diff --git a/frontend/src/app/DocumentsRoute.tsx b/frontend/src/app/DocumentsRoute.tsx
index 35bc944..3a73233 100644
--- a/frontend/src/app/DocumentsRoute.tsx
+++ b/frontend/src/app/DocumentsRoute.tsx
@@ -77,7 +77,7 @@ const DocumentsRouteContent: React.FC = () => {
   } = usePanelManager();
 
   const safeSidebarProps = useMemo<Record<string, unknown>>(
-    () => (sidebarProps && typeof sidebarProps === 'object' ? sidebarProps : {}),
+    () => (sidebarProps && Object(sidebarProps) === sidebarProps ? sidebarProps : {}),
     [sidebarProps],
   );
 
diff --git a/frontend/src/app/UploadQueueOverlay.tsx b/frontend/src/app/UploadQueueOverlay.tsx
index 7fbd272..5522621 100644
--- a/frontend/src/app/UploadQueueOverlay.tsx
+++ b/frontend/src/app/UploadQueueOverlay.tsx
@@ -112,7 +112,7 @@ const UploadQueueOverlay = ({ queue = [], onClearQueue }: UploadQueueOverlayProp
             <button
               type="button"
               className="icon-button ghost"
-              onClick={() => setCollapsed((value) => !value)}
+              onClick={() => setCollapsed(!collapsed)}
               aria-label={collapsed ? 'Expand upload queue' : 'Collapse upload queue'}
             >
               {collapsed ? <BottombarExpandIcon size={16} /> : <BottombarCollapseIcon size={16} />}
diff --git a/frontend/src/app/appLayoutUtils.ts b/frontend/src/app/appLayoutUtils.ts
index 041b2a8..362639c 100644
--- a/frontend/src/app/appLayoutUtils.ts
+++ b/frontend/src/app/appLayoutUtils.ts
@@ -16,15 +16,15 @@ export const resolveApiPath = (path = '') => path;
 const makeRowKey = (type, id) =>
   id ? `${type}${ROW_KEY_SEPARATOR}${id}` : `${type}${ROW_KEY_SEPARATOR}`;
 
-const getRowType = (key) => (
-  typeof key?.split === 'function' ? key.split(ROW_KEY_SEPARATOR, 1)[0] : ''
-);
+const normalizeRowKey = (key: string | number | null | undefined) => String(key ?? '');
+
+const getRowType = (key) => normalizeRowKey(key).split(ROW_KEY_SEPARATOR, 1)[0] ?? '';
 
 export const getRowId = (key) => {
-  if (typeof key?.indexOf !== 'function' || typeof key?.slice !== 'function') return '';
-  const separatorIndex = key.indexOf(ROW_KEY_SEPARATOR);
-  if (separatorIndex === -1) return key;
-  return key.slice(separatorIndex + 1);
+  const normalized = normalizeRowKey(key);
+  const separatorIndex = normalized.indexOf(ROW_KEY_SEPARATOR);
+  if (separatorIndex === -1) return normalized;
+  return normalized.slice(separatorIndex + 1);
 };
 
 export const isDocumentRowKey = (key) => getRowType(key) === DOCUMENT_ROW_PREFIX;
diff --git a/frontend/src/app/appState.tsx b/frontend/src/app/appState.tsx
index ea15f34..e728128 100644
--- a/frontend/src/app/appState.tsx
+++ b/frontend/src/app/appState.tsx
@@ -43,7 +43,7 @@ type AppAction =
   | { type: 'RESET_ERROR' }
   | { type: 'SET_TENANTS'; tenants: Tenant[] };
 
-const storage = typeof window !== 'undefined' ? window.sessionStorage : null;
+const storage = window.sessionStorage;
 
 const STORED_TOKEN = storage?.getItem('papercrate_token') ?? '';
 let STORED_TENANT: Tenant = null;
diff --git a/frontend/src/app/useDocumentPreview.ts b/frontend/src/app/useDocumentPreview.ts
index 4d173ac..3d119c5 100644
--- a/frontend/src/app/useDocumentPreview.ts
+++ b/frontend/src/app/useDocumentPreview.ts
@@ -149,7 +149,7 @@ const useDocumentPreview = ({
 
       const existing = previewEntries.get(documentId) || null;
       const now = Date.now();
-      const expiresAt = typeof existing?.expiresAt === 'number' ? existing.expiresAt : null;
+      const expiresAt = Number.isFinite(existing?.expiresAt) ? Number(existing?.expiresAt) : null;
       if (!force && existing && (!expiresAt || expiresAt > now)) {
         return existing;
       }
diff --git a/frontend/src/app/useManagementModals.tsx b/frontend/src/app/useManagementModals.tsx
index 3eb8207..461bd24 100644
--- a/frontend/src/app/useManagementModals.tsx
+++ b/frontend/src/app/useManagementModals.tsx
@@ -137,7 +137,7 @@ export const useManagementModals = ({
 
   const handleCorrespondentCreateSafe = useCallback<CorrespondentsPanelProps['onCreate']>(
     async (payload) => {
-      if (typeof onCorrespondentCreate !== 'function') {
+      if (!onCorrespondentCreate) {
         return undefined;
       }
       return onCorrespondentCreate(payload) ?? undefined;
@@ -147,7 +147,7 @@ export const useManagementModals = ({
 
   const handleCorrespondentUpdateSafe = useCallback<CorrespondentsPanelProps['onUpdate']>(
     async (id, payload) => {
-      if (typeof onCorrespondentUpdate !== 'function') {
+      if (!onCorrespondentUpdate) {
         return;
       }
       await onCorrespondentUpdate(id, payload);
@@ -157,7 +157,7 @@ export const useManagementModals = ({
 
   const handleCorrespondentDeleteSafe = useCallback<CorrespondentsPanelProps['onDelete']>(
     async (id) => {
-      if (typeof onCorrespondentDelete !== 'function') {
+      if (!onCorrespondentDelete) {
         return;
       }
       await onCorrespondentDelete(id);
diff --git a/frontend/src/app/useWorkspaceSelection.ts b/frontend/src/app/useWorkspaceSelection.ts
index 1823b71..fe22d7b 100644
--- a/frontend/src/app/useWorkspaceSelection.ts
+++ b/frontend/src/app/useWorkspaceSelection.ts
@@ -76,9 +76,9 @@ export const useWorkspaceSelection = ({
 
   const selectEntry = useCallback(
     (entry: SelectionEntry | string | null, event?: unknown) => {
-      const rowKey = typeof entry === 'string'
-        ? entry
-        : entry?.rowKey ?? null;
+      const rowKey = entry && Object(entry) === entry
+        ? (entry as SelectionEntry).rowKey ?? null
+        : (entry as string | null);
       if (!rowKey) return;
       handleEntrySelection(rowKey, event);
     },
diff --git a/frontend/src/asset_manager.ts b/frontend/src/asset_manager.ts
index ea87889..94f030a 100644
--- a/frontend/src/asset_manager.ts
+++ b/frontend/src/asset_manager.ts
@@ -81,15 +81,16 @@ const mergeAssetObjects = (
   const merged = new Map<number, AssetObject>();
 
   normalizeAssetObjects(existingObjects).forEach((entry) => {
-    if (typeof entry.ordinal === 'number') {
-      merged.set(entry.ordinal, { ...entry });
+    if (Number.isFinite(entry.ordinal)) {
+      merged.set(entry.ordinal as number, { ...entry });
     }
   });
 
   normalizeAssetObjects(incomingObjects).forEach((entry) => {
-    if (typeof entry.ordinal === 'number') {
-      const current = merged.get(entry.ordinal) || {};
-      merged.set(entry.ordinal, { ...current, ...entry });
+    if (Number.isFinite(entry.ordinal)) {
+      const ordinal = entry.ordinal as number;
+      const current = merged.get(ordinal) || {};
+      merged.set(ordinal, { ...current, ...entry });
     }
   });
 
@@ -198,23 +199,23 @@ export const resolveDocumentAssetUrl = (
   if (!doc || !type) {
     return null;
   }
-  const asset = typeof getAsset === 'function' ? getAsset(doc, type) : null;
+  const asset = getAsset ? getAsset(doc, type) : null;
   if (!asset) {
     return null;
   }
   const view = createAssetView(asset);
   const object = view.getObject(objectOrdinal);
   const url = object?.url || (objectOrdinal === 1 ? view.getPrimaryUrl() : null);
-  const expiresAt = typeof object?.expires_at === 'number'
-    ? object.expires_at
-    : objectOrdinal === 1 && typeof asset.expiresAt === 'number'
-      ? asset.expiresAt
+  const expiresAt = Number.isFinite(object?.expires_at)
+    ? Number(object?.expires_at)
+    : objectOrdinal === 1 && Number.isFinite(asset.expiresAt)
+      ? Number(asset.expiresAt)
       : null;
   const now = Date.now();
   if (url && (!expiresAt || expiresAt > now)) {
     return url;
   }
-  if (doc.id && asset.id && typeof ensureAssetUrl === 'function') {
+  if (doc.id && asset.id && ensureAssetUrl) {
     const force = Boolean(url && expiresAt && expiresAt <= now);
     const options: { force: boolean; start?: number | null; limit?: number | null; [key: string]: unknown } = {
       force,
@@ -397,7 +398,7 @@ class AssetManager {
 
     const baseAsset = this.assetCache.get(asset.id) || asset;
     const view = createAssetView(baseAsset);
-    const assetExpiresAt = typeof baseAsset.expiresAt === 'number' ? baseAsset.expiresAt : null;
+    const assetExpiresAt = Number.isFinite(baseAsset.expiresAt) ? Number(baseAsset.expiresAt) : null;
     const now = Date.now();
 
     const isOrdinalSatisfied = (ordinal) => {
@@ -408,8 +409,8 @@ class AssetManager {
       if (!object.url) {
         return false;
       }
-      if (typeof object.expires_at === 'number') {
-        return object.expires_at > now;
+      if (Number.isFinite(object.expires_at)) {
+        return Number(object.expires_at) > now;
       }
       if (ordinal === 1 && baseAsset.url && (!assetExpiresAt || assetExpiresAt > now)) {
         return true;
@@ -460,8 +461,8 @@ class AssetManager {
         const combined = { ...cachedEntry, ...asset, ...data, objects: mergedObjects };
         const view = createAssetView(combined);
         const primaryObject = view.getPrimaryObject();
-        const expiresAt = typeof primaryObject?.expires_at === 'number'
-          ? primaryObject.expires_at
+        const expiresAt = Number.isFinite(primaryObject?.expires_at)
+          ? Number(primaryObject?.expires_at)
           : Date.now() + this.assetPresignTtlMs;
         const cardinality = (() => {
           const reported = Number(data.cardinality ?? asset.cardinality ?? cachedEntry?.cardinality);
diff --git a/frontend/src/correspondents/CorrespondentsPanel.tsx b/frontend/src/correspondents/CorrespondentsPanel.tsx
index ee599b4..ef3f457 100644
--- a/frontend/src/correspondents/CorrespondentsPanel.tsx
+++ b/frontend/src/correspondents/CorrespondentsPanel.tsx
@@ -125,7 +125,7 @@ function CorrespondentsPanel({
     if (!usage) {
       return '0';
     }
-    const total = typeof usage.total === 'number' ? usage.total : 0;
+    const total = Number.isFinite(usage.total) ? Number(usage.total) : 0;
     return total.toString();
   }, []);
 
diff --git a/frontend/src/desktop/DesktopWorkspace.tsx b/frontend/src/desktop/DesktopWorkspace.tsx
index b33c0e1..9bc6ebf 100644
--- a/frontend/src/desktop/DesktopWorkspace.tsx
+++ b/frontend/src/desktop/DesktopWorkspace.tsx
@@ -419,14 +419,7 @@ const DesktopWorkspace: React.FC<DesktopWorkspaceProps> = ({
 
     commitSize();
 
-    if (typeof window.ResizeObserver === 'undefined') {
-      window.addEventListener('resize', commitSize);
-      return () => {
-        window.removeEventListener('resize', commitSize);
-      };
-    }
-
-    const observer = new window.ResizeObserver(commitSize);
+    const observer = new ResizeObserver(commitSize);
     observer.observe(container);
     return () => observer.disconnect();
   }, [engine]);
diff --git a/frontend/src/desktop/db.ts b/frontend/src/desktop/db.ts
index b7c7b79..85ca546 100644
--- a/frontend/src/desktop/db.ts
+++ b/frontend/src/desktop/db.ts
@@ -10,13 +10,7 @@ const openDatabase = (): Promise<IDBDatabase> => {
   }
 
   currentDbPromise.value = new Promise((resolve, reject) => {
-    const dbApi = typeof window !== 'undefined' ? window.indexedDB : null;
-    if (!dbApi) {
-      reject(new Error('IndexedDB not available'));
-      return;
-    }
-
-    const request = dbApi.open(DB_NAME, DB_VERSION);
+    const request = window.indexedDB.open(DB_NAME, DB_VERSION);
 
     request.onupgradeneeded = () => {
       const db = request.result;
diff --git a/frontend/src/desktop/tags/useDeskTagInteractions.js b/frontend/src/desktop/tags/useDeskTagInteractions.js
index e5fa6d6..cc064b8 100644
--- a/frontend/src/desktop/tags/useDeskTagInteractions.js
+++ b/frontend/src/desktop/tags/useDeskTagInteractions.js
@@ -141,7 +141,7 @@ export const useDeskTagInteractions = ({
 
       const { x: pointerX, y: pointerY } = getPointerPosition(event, { fallbackToPage: false });
       const { clone, offsetX, offsetY } = createDragPreview(event.currentTarget, pointerX, pointerY) || {};
-      if (clone && typeof event.dataTransfer.setDragImage === 'function') {
+      if (clone) {
         event.dataTransfer.setDragImage(clone, offsetX || 0, offsetY || 0);
       }
 
diff --git a/frontend/src/desktop/useDocumentDrag.ts b/frontend/src/desktop/useDocumentDrag.ts
index e2dad95..de07768 100644
--- a/frontend/src/desktop/useDocumentDrag.ts
+++ b/frontend/src/desktop/useDocumentDrag.ts
@@ -165,13 +165,9 @@ const getEventTargetElement = (event?: PointerEventLike | null): Element | null
   if (!event) {
     return null;
   }
-  const ElementCtor = typeof window !== 'undefined' ? window.Element : null;
-  if (!ElementCtor) {
-    return null;
-  }
   const nativeEvent = 'nativeEvent' in event ? (event as ReactPointerEvent).nativeEvent : null;
   const candidate = (event.target as Element | null) || (nativeEvent ? (nativeEvent.target as Element | null) : null);
-  return candidate instanceof ElementCtor ? candidate : null;
+  return candidate instanceof Element ? candidate : null;
 };
 
 const useDocumentDrag = (options: UseDocumentDragOptions) => {
diff --git a/frontend/src/desktop/workspaceEngine.ts b/frontend/src/desktop/workspaceEngine.ts
index 505b6f6..abb85ae 100644
--- a/frontend/src/desktop/workspaceEngine.ts
+++ b/frontend/src/desktop/workspaceEngine.ts
@@ -618,7 +618,7 @@ export class WorkspaceEngine {
 
   updateConfig({ allowLayoutPersistence, tenantId, viewId }: WorkspaceEngineOptions): void {
     const allowChanged =
-      typeof allowLayoutPersistence === 'boolean'
+      allowLayoutPersistence !== undefined
       && allowLayoutPersistence !== this.allowLayoutPersistence;
     const tenantChanged = tenantId !== undefined && tenantId !== this.tenantId;
     const viewChanged = viewId !== undefined && viewId !== this.viewId;
@@ -682,15 +682,11 @@ export class WorkspaceEngine {
   }
 
   setEnsureDocumentSize(fn: EnsureDocumentSize): void {
-    if (typeof fn === 'function') {
-      this.ensureDocumentSize = fn;
-    }
+    this.ensureDocumentSize = fn;
   }
 
   setResolveBaseMetrics(fn: ResolveBaseMetrics): void {
-    if (typeof fn === 'function') {
-      this.resolveBaseMetrics = fn;
-    }
+    this.resolveBaseMetrics = fn;
   }
 
   setItemRefs(ref: ItemRefs | null | undefined): void {
@@ -782,18 +778,14 @@ export class WorkspaceEngine {
 
   updateLayoutEntry(
     docId: string | number | null,
-    updater:
-      | LayoutEntry
-      | null
-      | undefined
-      | ((previous: LayoutEntry | null) => LayoutEntry | null | undefined),
+    updater: (previous: LayoutEntry | null) => LayoutEntry | null | undefined,
   ): void {
     if (docId == null) {
       return;
     }
     const key = String(docId);
     const previous = this.layout.get(key) || null;
-    const next = typeof updater === 'function' ? updater(previous) : updater;
+    const next = updater(previous);
     if (!next) {
       this.layout.delete(key);
     } else {
@@ -939,7 +931,9 @@ export class WorkspaceEngine {
     let angularVelocity = simulationState.angularVelocity + torqueAcceleration * dt;
     angularVelocity = clamp(angularVelocity, -MAX_ANGULAR_VELOCITY, MAX_ANGULAR_VELOCITY);
 
-    const dampingConstant = typeof dampingOverride === 'number' ? dampingOverride : ANGULAR_DAMPING;
+    const dampingConstant = Number.isFinite(dampingOverride)
+      ? Number(dampingOverride)
+      : ANGULAR_DAMPING;
     const dampingFactor = Math.exp(-dampingConstant * dt);
     angularVelocity *= dampingFactor;
 
@@ -1154,8 +1148,12 @@ export class WorkspaceEngine {
       if (existing) {
         const defaultCenterX = (minCenterX + maxCenterX) / 2;
         const defaultCenterY = (minCenterY + maxCenterY) / 2;
-        const prevCenterX = typeof existing.centerX === 'number' ? existing.centerX : defaultCenterX;
-        const prevCenterY = typeof existing.centerY === 'number' ? existing.centerY : defaultCenterY;
+        const prevCenterX = Number.isFinite(existing.centerX)
+          ? Number(existing.centerX)
+          : defaultCenterX;
+        const prevCenterY = Number.isFinite(existing.centerY)
+          ? Number(existing.centerY)
+          : defaultCenterY;
         const centerX = clamp(prevCenterX, minCenterX, maxCenterX);
         const centerY = clamp(prevCenterY, minCenterY, maxCenterY);
         const rotation = existing.rotation ?? 0;
@@ -1201,9 +1199,6 @@ export class WorkspaceEngine {
 
   recalcVisibleDocIds(): void {
     const ensureSize = this.ensureDocumentSize;
-    if (typeof ensureSize !== 'function') {
-      return;
-    }
 
     const layoutMap = this.layout;
     const canvasWidth = this.canvasSize.width || DESK_DEFAULT_CANVAS_WIDTH;
@@ -1418,7 +1413,7 @@ export const useWorkspaceSnapshot = (
   useSyncExternalStoreHook: UseSyncExternalStoreHook,
 ): WorkspaceSnapshot => {
   const useSyncExternalStore = useSyncExternalStoreHook;
-  if (typeof useSyncExternalStore !== 'function') {
+  if (!useSyncExternalStore) {
     throw new Error('useWorkspaceSnapshot requires useSyncExternalStore hook');
   }
   return useSyncExternalStore(
diff --git a/frontend/src/detail/PreviewZoomOverlay.tsx b/frontend/src/detail/PreviewZoomOverlay.tsx
index 54dc09d..c5a70f3 100644
--- a/frontend/src/detail/PreviewZoomOverlay.tsx
+++ b/frontend/src/detail/PreviewZoomOverlay.tsx
@@ -28,7 +28,7 @@ const PreviewZoomOverlay: React.FC<PreviewZoomOverlayProps> = ({
   display = null,
   onClose = noop,
 }) => {
-  const portalTarget = typeof document !== 'undefined' ? document.body : null;
+  const portalTarget = document.body;
   const [isNativeScale, setIsNativeScale] = useState(false);
   const [naturalSize, setNaturalSize] = useState<NaturalSize>({ width: null, height: null });
   const [renderBackdrop, setRenderBackdrop] = useState(false);
@@ -135,9 +135,6 @@ const PreviewZoomOverlay: React.FC<PreviewZoomOverlayProps> = ({
   }, [open, isNativeScale, naturalSize.width, naturalSize.height]);
 
   useEffect(() => {
-    if (typeof document === 'undefined') {
-      return;
-    }
     if (!open) {
       previouslyFocusedRef.current?.focus?.();
       previouslyFocusedRef.current = null;
@@ -247,7 +244,7 @@ const PreviewZoomOverlay: React.FC<PreviewZoomOverlayProps> = ({
     toggleZoomAtPoint(event.clientX ?? 0, event.clientY ?? 0);
   };
 
-  if (!renderBackdrop || !activeDisplay?.url || !portalTarget) {
+  if (!renderBackdrop || !activeDisplay?.url) {
     return null;
   }
 
diff --git a/frontend/src/documents/DocumentInfoPanel.tsx b/frontend/src/documents/DocumentInfoPanel.tsx
index f359f3d..4e19842 100644
--- a/frontend/src/documents/DocumentInfoPanel.tsx
+++ b/frontend/src/documents/DocumentInfoPanel.tsx
@@ -88,13 +88,14 @@ const DocumentInfoPanel: React.FC<DocumentInfoPanelProps> = ({
 
   const contentConfig = contentConfigProp || null;
   const contentEnabled = Boolean(contentConfig && (contentConfig.enabled ?? true));
+  const loadContent = contentConfig?.loadContent ?? null;
   const showContentTab = Boolean(contentConfig && (contentConfig.forceDisplay ?? contentEnabled));
 
   const [contentState, setContentState] = useState<ContentState | null>(() => {
     if (!contentConfig) {
       return null;
     }
-    if (!contentEnabled || typeof contentConfig.loadContent !== 'function') {
+    if (!contentEnabled || !loadContent) {
       return { status: contentEnabled ? 'idle' : 'unavailable', data: null, error: null };
     }
     return { status: 'idle', data: null, error: null };
@@ -106,7 +107,7 @@ const DocumentInfoPanel: React.FC<DocumentInfoPanelProps> = ({
       return undefined;
     }
 
-    if (!contentEnabled || typeof contentConfig.loadContent !== 'function') {
+    if (!contentEnabled || !loadContent) {
       setContentState({ status: contentEnabled ? 'idle' : 'unavailable', data: null, error: null });
       return undefined;
     }
@@ -116,7 +117,7 @@ const DocumentInfoPanel: React.FC<DocumentInfoPanelProps> = ({
 
     setContentState({ status: 'loading', data: null, error: null });
 
-    Promise.resolve(contentConfig.loadContent({ signal: controller.signal }))
+    Promise.resolve(loadContent({ signal: controller.signal }))
       .then((result) => {
         if (cancelled) {
           return;
@@ -143,7 +144,7 @@ const DocumentInfoPanel: React.FC<DocumentInfoPanelProps> = ({
       controller.abort();
       contentConfig.onCancel?.();
     };
-  }, [contentConfig, contentEnabled, showContentTab, document?.id, resetKey]);
+  }, [contentConfig, contentEnabled, showContentTab, document?.id, resetKey, loadContent]);
 
   const renderSummarySection = useCallback(() => (
     <DocumentSummarySection
@@ -345,10 +346,10 @@ const DocumentInfoPanel: React.FC<DocumentInfoPanelProps> = ({
     if (!tab) {
       return null;
     }
-    if (typeof tab.render === 'function') {
-      return tab.render(context);
+    if (!tab.render) {
+      return null;
     }
-    return null;
+    return tab.render(context);
   };
 
   const isControlled = controlledActiveTab !== undefined && controlledActiveTab !== null;
diff --git a/frontend/src/documents/DocumentSummarySection.tsx b/frontend/src/documents/DocumentSummarySection.tsx
index 2dd8230..7d6410d 100644
--- a/frontend/src/documents/DocumentSummarySection.tsx
+++ b/frontend/src/documents/DocumentSummarySection.tsx
@@ -9,6 +9,7 @@ import {
   toIssuedTimestamp,
 } from '../utils/date';
 import { describeDocumentSummary } from './documentSummary';
+import { isPlainObject } from '../utils/typeGuards';
 
 type Identifier = string | number;
 
@@ -110,43 +111,33 @@ interface QuickAddEntry {
   original: QuickAddOption | string;
 }
 
-const resolveOptionName = (source: unknown): string => {
+const resolveOptionName = (source?: QuickAddOption | string | null): string => {
   if (!source) {
     return '';
   }
-  if (typeof source === 'string') {
-    return source.trim();
+  if (isPlainObject(source)) {
+    const raw = source.name ?? source.label ?? '';
+    return `${raw}`.trim();
   }
-  if (typeof source === 'object') {
-    const candidate = source as { name?: string; label?: string; trim?: () => string };
-    if (typeof candidate.name === 'string' && candidate.name.trim()) {
-      return candidate.name.trim();
-    }
-    if (typeof candidate.label === 'string' && candidate.label.trim()) {
-      return candidate.label.trim();
-    }
-    if (typeof candidate.trim === 'function') {
-      const viaTrim = candidate.trim();
-      if (typeof viaTrim === 'string' && viaTrim.trim()) {
-        return viaTrim.trim();
-      }
-    }
-  }
-  return '';
+  return `${source}`.trim();
 };
 
 const normalizeQuickAddOption = (option: QuickAddOption | string | null | undefined): QuickAddEntry | null => {
   if (option == null) {
     return null;
   }
-  const isObject = typeof option === 'object';
-  const labelSource = isObject ? option.label ?? option.name ?? '' : option;
-  const label = typeof labelSource === 'string' ? labelSource.trim() : '';
+  const label = (() => {
+    if (isPlainObject(option)) {
+      const sourceLabel = option.label ?? option.name ?? '';
+      return `${sourceLabel}`.trim();
+    }
+    return `${option}`.trim();
+  })();
   if (!label) {
     return null;
   }
   return {
-    id: isObject && option.id ? option.id : label,
+    id: isPlainObject(option) && option.id ? option.id : label,
     label,
     original: option,
   };
@@ -170,10 +161,7 @@ export const TagSection: React.FC<TagSectionProps> = ({
   const handleSelect = useCallback(
     (option: { label?: string; name?: string } | string | null) => {
       if (!onAdd) return;
-      const labelSource = option && typeof option === 'object'
-        ? option.label ?? option.name ?? ''
-        : option;
-      const label = typeof labelSource === 'string' ? labelSource.trim() : '';
+      const label = resolveOptionName(option as QuickAddOption | string | null);
       if (!label) {
         return;
       }
@@ -372,13 +360,13 @@ export const CorrespondentSection: React.FC<CorrespondentSectionProps> = ({
       if (!onAdd || !item) {
         return;
       }
-      const source = item.payload ?? item;
+      const source = (item.payload ?? item) as QuickAddOption | string | null;
       const resolvedName = resolveOptionName(source);
       if (!resolvedName) {
         return;
       }
-      const payload = (source && typeof source === 'object')
-        ? { ...(source as Record<string, unknown>), name: resolvedName }
+      const payload = isPlainObject(source)
+        ? { ...source, name: resolvedName }
         : { id: null, name: resolvedName };
       onAdd({ name: resolvedName, option: payload, input: null });
     },
diff --git a/frontend/src/documents/DocumentThumbnailImage.tsx b/frontend/src/documents/DocumentThumbnailImage.tsx
index 5209e45..38681a1 100644
--- a/frontend/src/documents/DocumentThumbnailImage.tsx
+++ b/frontend/src/documents/DocumentThumbnailImage.tsx
@@ -38,12 +38,12 @@ const useLazyVisibility = (
       return undefined;
     }
 
-    if (typeof window === 'undefined' || !('IntersectionObserver' in window)) {
+    if (!window.IntersectionObserver) {
       setIsVisible(true);
       return undefined;
     }
 
-    const observer = new IntersectionObserver(
+    const observer = new window.IntersectionObserver(
       (entries) => {
         entries.forEach((entry) => {
           if (entry.isIntersecting) {
diff --git a/frontend/src/documents/DocumentsGrid.tsx b/frontend/src/documents/DocumentsGrid.tsx
index 0a9e2cb..30ab662 100644
--- a/frontend/src/documents/DocumentsGrid.tsx
+++ b/frontend/src/documents/DocumentsGrid.tsx
@@ -175,7 +175,7 @@ const DocumentsGrid: React.FC<DocumentsGridProps> = ({
         const classes = ['document-card', 'folder-card'];
         if (isDraggingFolder) classes.push('is-dragging');
         if (isSelectedFolder) classes.push('selected');
-        const canRenameFolder = folder.id !== 'root' && typeof onFolderRename === 'function';
+        const canRenameFolder = Boolean(onFolderRename) && folder.id !== 'root';
         const isFolderEditing = editingFolderId === folder.id;
         const folderDraftValue = isFolderEditing ? folderDraft : folder.name;
         const trimmedFolderDraft = isFolderEditing ? folderDraft.trim() : '';
diff --git a/frontend/src/documents/DocumentsList.tsx b/frontend/src/documents/DocumentsList.tsx
index 5498f1f..7bd09df 100644
--- a/frontend/src/documents/DocumentsList.tsx
+++ b/frontend/src/documents/DocumentsList.tsx
@@ -186,7 +186,7 @@ const DocumentsList: React.FC<DocumentsListProps> = ({
           const isDraggingFolder = draggedFolderId === folder.id;
           const isSelectedFolder = selectedFolderIdsSet?.has(folder.id);
           const rowKey = `folder:${folder.id}`;
-          const canRenameFolder = folder.id !== 'root' && typeof onFolderRename === 'function';
+          const canRenameFolder = Boolean(onFolderRename) && folder.id !== 'root';
           const isFolderEditing = editingFolderId === folder.id;
           const folderDraftValue = isFolderEditing ? folderDraft : folder.name;
           const trimmedFolderDraft = isFolderEditing ? folderDraft.trim() : '';
diff --git a/frontend/src/documents/SelectionAssignmentMenu.tsx b/frontend/src/documents/SelectionAssignmentMenu.tsx
index 4942281..945b2db 100644
--- a/frontend/src/documents/SelectionAssignmentMenu.tsx
+++ b/frontend/src/documents/SelectionAssignmentMenu.tsx
@@ -64,12 +64,14 @@ const normalizeItems = (items?: SelectionAssignmentMenuItem[]): NormalizedSelect
         : item.state === 'partial'
           ? 'partial'
           : 'none';
+      const numericCount = Number.isFinite(item.count) ? Number(item.count) : null;
+      const numericTotal = Number.isFinite(item.total) ? Number(item.total) : null;
       return {
         id: item.id ?? trimmedLabel,
         label: trimmedLabel,
         state,
-        count: typeof item.count === 'number' ? item.count : null,
-        total: typeof item.total === 'number' ? item.total : null,
+        count: numericCount,
+        total: numericTotal,
         payload: item.payload ?? item,
       };
     })
@@ -160,7 +162,7 @@ const SelectionAssignmentMenu: React.FC<SelectionAssignmentMenuProps> = ({
 
   const handleToggle = useCallback(
     async (item: NormalizedSelectionAssignmentItem) => {
-      if (!item || typeof onToggle !== 'function') {
+      if (!item || !onToggle) {
         return;
       }
       setPending(true);
@@ -179,7 +181,7 @@ const SelectionAssignmentMenu: React.FC<SelectionAssignmentMenuProps> = ({
   const handleCreate = useCallback(
     async (event: React.FormEvent<HTMLFormElement>) => {
       event?.preventDefault?.();
-      if (typeof onCreate !== 'function') {
+      if (!onCreate) {
         return;
       }
       const value = query.trim();
diff --git a/frontend/src/documents/SelectionFloatingActions.tsx b/frontend/src/documents/SelectionFloatingActions.tsx
index 04f5f59..7dd97a6 100644
--- a/frontend/src/documents/SelectionFloatingActions.tsx
+++ b/frontend/src/documents/SelectionFloatingActions.tsx
@@ -89,6 +89,13 @@ const normalizeDocumentList = (selectedIds?: SelectedIdList): DocumentId[] =>
     ? selectedIds.filter((value): value is DocumentId => value !== null && value !== undefined)
     : [];
 
+const isRecord = (value: unknown): value is Record<string, unknown> => value != null && Object(value) === value;
+
+const splitLabelSegments = (input: unknown): string[] => {
+  const text = `${input ?? ''}`.trim();
+  return text ? text.split('/') : [];
+};
+
 const buildFolderTreeOptions = (tree?: FolderTreeNode[] | null): SelectionAssignmentMenuItem[] => {
   const entries: SelectionAssignmentMenuItem[] = [];
 
@@ -346,7 +353,9 @@ const SelectionFloatingActions: React.FC<SelectionFloatingActionsProps> = ({
 
   const documentCount = documentIdList.length;
   const folderCount = folderIdList.length;
-  const totalCount = typeof selectionCount === 'number' ? selectionCount : documentCount + folderCount;
+  const totalCount = Number.isFinite(selectionCount)
+    ? Number(selectionCount)
+    : documentCount + folderCount;
 
   const selectedDocuments = useMemo<DocumentLike[]>(() => {
     if (!documentIdList.length || !(documentLookupMap instanceof Map)) {
@@ -370,7 +379,7 @@ const SelectionFloatingActions: React.FC<SelectionFloatingActionsProps> = ({
           return null;
         }
         const label = option?.label || option?.payload?.label || option?.name || String(id);
-        const segments = typeof label === 'string' ? label.split('/') : [label];
+        const segments = splitLabelSegments(label);
         const depth = Math.max(segments.length - 1, 0);
         return {
           id,
@@ -399,7 +408,7 @@ const SelectionFloatingActions: React.FC<SelectionFloatingActionsProps> = ({
 
   const renderFolderLabel = useCallback((item: SelectionAssignmentMenuItem) => {
     const payload = (item?.payload as { segments?: string[]; depth?: number }) || {};
-    const segments = payload.segments || (typeof item.label === 'string' ? item.label.split('/') : []);
+    const segments = payload.segments || splitLabelSegments(item.label);
     const depth = payload.depth ?? Math.max(segments.length - 1, 0);
     const clampedDepth = Math.min(depth, 6);
     const indentWidth = clampedDepth > 0 ? clampedDepth * 0.9 : 0;
@@ -481,12 +490,12 @@ const SelectionFloatingActions: React.FC<SelectionFloatingActionsProps> = ({
 
   const handleMoveSelectionToFolder = useCallback(
     async (option: unknown) => {
-      if (!documentIdList.length || typeof onMoveDocumentsToFolder !== 'function') {
+      if (!documentIdList.length || !onMoveDocumentsToFolder) {
         return;
       }
       const candidate = option as { id?: DocumentId; value?: DocumentId } | DocumentId | null | undefined;
-      const value = typeof candidate === 'object'
-        ? candidate?.id ?? candidate?.value ?? null
+      const value = isRecord(candidate)
+        ? (candidate?.id ?? candidate?.value ?? null)
         : candidate;
       if (!value && value !== 0) {
         return;
@@ -506,7 +515,7 @@ const SelectionFloatingActions: React.FC<SelectionFloatingActionsProps> = ({
 
   const showPrimaryButtons = Boolean(onBulkReanalyze || onDeleteSelection || onClearSelection);
 
-  const moveMenu = typeof onMoveDocumentsToFolder === 'function' ? (
+  const moveMenu = onMoveDocumentsToFolder ? (
     <SelectionAssignmentMenu
       label="Move"
       triggerContent={(
@@ -534,7 +543,7 @@ const SelectionFloatingActions: React.FC<SelectionFloatingActionsProps> = ({
 
   const primaryButtons = showPrimaryButtons ? (
     <div className="panel-floating__buttons">
-      {typeof onBulkReanalyze === 'function' ? (
+      {onBulkReanalyze ? (
         <button
           type="button"
           className="icon-button panel-floating-actions__button"
@@ -546,7 +555,7 @@ const SelectionFloatingActions: React.FC<SelectionFloatingActionsProps> = ({
           <AnalyzeIcon className="icon-inline" />
         </button>
       ) : null}
-      {typeof onDeleteSelection === 'function' ? (
+      {onDeleteSelection ? (
         <button
           type="button"
           className="icon-button danger panel-floating-actions__button"
@@ -557,7 +566,7 @@ const SelectionFloatingActions: React.FC<SelectionFloatingActionsProps> = ({
           <TrashIcon className="icon-inline" />
         </button>
       ) : null}
-      {typeof onClearSelection === 'function' ? (
+      {onClearSelection ? (
         <button
           type="button"
           className="icon-button panel-floating-actions__button"
diff --git a/frontend/src/documents/documentSummary.ts b/frontend/src/documents/documentSummary.ts
index 6829d35..63414be 100644
--- a/frontend/src/documents/documentSummary.ts
+++ b/frontend/src/documents/documentSummary.ts
@@ -61,16 +61,11 @@ export interface DocumentSummary {
 
 const coercePageCount = (metadata: DocumentMetadata | null | undefined): number | null => {
   const raw = metadata?.page_count;
-  if (typeof raw === 'number') {
-    return Number.isFinite(raw) && raw >= 0 ? raw : null;
+  if (raw == null || raw === '') {
+    return null;
   }
-  if (raw != null && raw !== '') {
-    const parsed = Number.parseInt(String(raw), 10);
-    if (Number.isFinite(parsed) && parsed >= 0) {
-      return parsed;
-    }
-  }
-  return null;
+  const parsed = Number.parseInt(String(raw), 10);
+  return Number.isFinite(parsed) && parsed >= 0 ? parsed : null;
 };
 
 const sanitizeArray = <T>(entries: (T | null | undefined)[] | null | undefined): T[] =>
diff --git a/frontend/src/documents/panel/DocumentsPanel.tsx b/frontend/src/documents/panel/DocumentsPanel.tsx
index b30aecc..9d3e22e 100644
--- a/frontend/src/documents/panel/DocumentsPanel.tsx
+++ b/frontend/src/documents/panel/DocumentsPanel.tsx
@@ -192,7 +192,7 @@ const DocumentsPanel: React.FC<DocumentsPanelProps> = ({
       if (!doc || !doc.id) {
         return;
       }
-      const previewAsset = typeof getDocumentAsset === 'function' ? getDocumentAsset(doc, 'preview') : null;
+      const previewAsset = getDocumentAsset(doc, 'preview');
       if (!previewAsset) {
         return;
       }
@@ -207,12 +207,8 @@ const DocumentsPanel: React.FC<DocumentsPanelProps> = ({
         return;
       }
       if (event) {
-        if (typeof event.preventDefault === 'function') {
-          event.preventDefault();
-        }
-        if (typeof event.stopPropagation === 'function') {
-          event.stopPropagation();
-        }
+        event.preventDefault();
+        event.stopPropagation();
       }
       if (event?.altKey) {
         handleDocumentPreviewZoom(doc);
@@ -489,16 +485,14 @@ const DocumentsPanel: React.FC<DocumentsPanelProps> = ({
 
   const handleDocumentClick = useCallback(
     (doc, event) => {
-      if (!doc || suppressDocumentClickRef.current) {
+      if (!doc || suppressDocumentClickRef.current || !onEntryPointer) {
         return;
       }
 
-      if (typeof onEntryPointer === 'function') {
-        onEntryPointer(
-          { type: EntryType.document, id: doc.id, key: `document:${doc.id}`, document: doc },
-          event,
-        );
-      }
+      onEntryPointer(
+        { type: EntryType.document, id: doc.id, key: `document:${doc.id}`, document: doc },
+        event,
+      );
     },
     [onEntryPointer],
   );
@@ -509,7 +503,7 @@ const DocumentsPanel: React.FC<DocumentsPanelProps> = ({
         return;
       }
 
-      if (typeof onEntryPointer === 'function') {
+      if (onEntryPointer) {
         onEntryPointer(
           { type: EntryType.folder, id: folder.id, key: `folder:${folder.id}`, folder },
           event,
diff --git a/frontend/src/documents/panel/DocumentsToolbar.tsx b/frontend/src/documents/panel/DocumentsToolbar.tsx
index d01b713..a7d35f7 100644
--- a/frontend/src/documents/panel/DocumentsToolbar.tsx
+++ b/frontend/src/documents/panel/DocumentsToolbar.tsx
@@ -49,7 +49,7 @@ export const createDocumentsTableHeaderActions = ({
     ? 'Sorting Z → A. Click to switch to ascending.'
     : 'Sorting A → Z. Click to switch to descending.';
 
-  const includeDescendantsToggle = isFilterActive && typeof onToggleIncludeDescendants === 'function'
+  const includeDescendantsToggle = isFilterActive && onToggleIncludeDescendants
     ? (
         <button
           type="button"
@@ -66,11 +66,11 @@ export const createDocumentsTableHeaderActions = ({
       )
     : null;
 
-  const sortControls = typeof onSortFieldChange === 'function'
+  const sortControls = onSortFieldChange
     ? (
         <div className="documents-actions__sort-group">
           <SortFieldQuickMenu sortField={sortField} onChange={onSortFieldChange} />
-          {typeof onSortDirectionToggle === 'function' ? (
+          {onSortDirectionToggle ? (
             <button
               type="button"
               className="icon-button documents-toolbar__toggle documents-sort__direction"
diff --git a/frontend/src/documents/tagTransfer.ts b/frontend/src/documents/tagTransfer.ts
index 8ea8e53..be9813b 100644
--- a/frontend/src/documents/tagTransfer.ts
+++ b/frontend/src/documents/tagTransfer.ts
@@ -85,12 +85,13 @@ type DragEventLike = DragEvent | DataTransfer | {
 };
 
 export const parseTagTransferPayload = (input: DataTransfer | DragEventLike | null): TagPayload | null => {
-    let dataTransfer: DataTransfer | null = null;
-  if (input) {
-    if (typeof DataTransfer !== 'undefined' && input instanceof DataTransfer) {
-      dataTransfer = input;
-    } else if (typeof input === 'object' && 'dataTransfer' in input && input.dataTransfer) {
-      dataTransfer = input.dataTransfer;
+  let dataTransfer: DataTransfer | null = null;
+  if (input instanceof DataTransfer) {
+    dataTransfer = input;
+  } else if (input && Object(input) === input && 'dataTransfer' in (input as Record<string, unknown>)) {
+    const candidate = (input as { dataTransfer?: DataTransfer | null }).dataTransfer;
+    if (candidate) {
+      dataTransfer = candidate;
     }
   }
   const raw = readTagTransferData(dataTransfer || null);
@@ -112,10 +113,11 @@ export const isTagTransferEvent = (event?: DragEventLike | null): boolean => {
     return false;
   }
   let types: DOMStringList | ReadonlyArray<string> | undefined;
-  if (typeof DataTransfer !== 'undefined' && event instanceof DataTransfer) {
+  if (event instanceof DataTransfer) {
     types = event.types;
-  } else if ('dataTransfer' in event && event.dataTransfer) {
-    types = event.dataTransfer.types;
+  } else if (Object(event) === event && 'dataTransfer' in (event as Record<string, unknown>)) {
+    const payload = (event as { dataTransfer?: DataTransfer | null }).dataTransfer;
+    types = payload?.types;
   }
   if (!types) {
     return false;
diff --git a/frontend/src/documents/useEntryPointer.ts b/frontend/src/documents/useEntryPointer.ts
index ad18f8b..6cd7a1f 100644
--- a/frontend/src/documents/useEntryPointer.ts
+++ b/frontend/src/documents/useEntryPointer.ts
@@ -9,7 +9,7 @@ export const isPrimaryPointerEvent = (event?: PointerEventLike | null): boolean
   if (!event) {
     return true;
   }
-  if (typeof event.button === 'number' && event.button !== 0) {
+  if (event.button !== 0) {
     return false;
   }
   const type = event?.type?.toLowerCase?.() ?? '';
diff --git a/frontend/src/hooks/documents/useCorrespondents.ts b/frontend/src/hooks/documents/useCorrespondents.ts
index 6630742..ded0130 100644
--- a/frontend/src/hooks/documents/useCorrespondents.ts
+++ b/frontend/src/hooks/documents/useCorrespondents.ts
@@ -53,7 +53,7 @@ const useCorrespondents = ({
       }
 
       const payload: Record<string, unknown> = {};
-      if (typeof changes?.name?.trim === 'function') {
+      if (changes?.name != null) {
         const trimmed = changes.name.trim();
         if (!trimmed) {
           throw new Error('Correspondent name cannot be empty.');
@@ -120,9 +120,7 @@ const useCorrespondents = ({
         await apiClient.delete(`/correspondents/${correspondentId}`);
         await refreshCorrespondents();
 
-        if (typeof mapDocumentCaches === 'function') {
-          mapDocumentCaches(stripFromDoc);
-        }
+        mapDocumentCaches?.(stripFromDoc);
 
         setStatusMessage('Correspondent deleted.', 'success');
         return true;
diff --git a/frontend/src/hooks/documents/useDocumentCorrespondentActions.ts b/frontend/src/hooks/documents/useDocumentCorrespondentActions.ts
index 830c0b4..053b09d 100644
--- a/frontend/src/hooks/documents/useDocumentCorrespondentActions.ts
+++ b/frontend/src/hooks/documents/useDocumentCorrespondentActions.ts
@@ -1,4 +1,5 @@
 import { useCallback, useMemo } from 'react';
+import { isPlainObject, isStringValue } from '../../utils/typeGuards';
 
 type ApiClient = {
   post: (path: string, body?: unknown) => Promise<{ data: unknown }>;
@@ -99,10 +100,10 @@ const useDocumentCorrespondentActions = ({
     if (!option) {
       return null;
     }
-    if (typeof option === 'object' && 'id' in option) {
+    if (isPlainObject(option) && 'id' in option) {
       return option as CorrespondentOption;
     }
-    if (typeof option === 'string') {
+    if (isStringValue(option)) {
       const trimmed = option.trim();
       if (trimmed) {
         return { id: null, name: trimmed };
diff --git a/frontend/src/hooks/documents/useDocumentDragHandlers.ts b/frontend/src/hooks/documents/useDocumentDragHandlers.ts
index cf03f02..de5ed80 100644
--- a/frontend/src/hooks/documents/useDocumentDragHandlers.ts
+++ b/frontend/src/hooks/documents/useDocumentDragHandlers.ts
@@ -1,5 +1,6 @@
 import { useCallback, useEffect, useRef } from 'react';
 import type { DragEvent } from 'react';
+import { isPlainObject, isFunctionValue } from '../../utils/typeGuards';
 
 type Identifier = string | number;
 type FolderIdentifier = Identifier | 'root';
@@ -144,11 +145,17 @@ const useDocumentDragHandlers = ({
           }
         } else {
           const payload = item.payload;
-          const folderId = (payload && typeof payload === 'object' && 'id' in payload)
-            ? (payload as { id?: FolderIdentifier }).id
-            : (typeof (payload as { trim?: () => string })?.trim === 'function'
-              ? (payload as { trim: () => string }).trim()
-              : null);
+          const folderId = (() => {
+            if (isPlainObject(payload) && 'id' in payload) {
+              return (payload as { id?: FolderIdentifier }).id ?? null;
+            }
+            const maybeTrim = (payload as { trim?: () => string })?.trim;
+            if (isFunctionValue(maybeTrim)) {
+              const nextValue = maybeTrim.call(payload);
+              return nextValue || null;
+            }
+            return null;
+          })();
           const rowEl = folderId
             ? (document.getElementById(`folder-row-${folderId}`)
               || document.getElementById(`folder-card-${folderId}`))
@@ -205,10 +212,9 @@ const useDocumentDragHandlers = ({
 
   const handleDocumentDragStart = useCallback(
     (event: DragEvent<HTMLElement>, documentOrId: DocumentLike | Identifier | null | undefined) => {
-      const documentId =
-        (documentOrId as DocumentLike)?.id ?? (typeof documentOrId === 'string' || typeof documentOrId === 'number'
-          ? documentOrId
-          : null);
+      const documentId: Identifier | null = Object(documentOrId) === documentOrId
+        ? (documentOrId as DocumentLike)?.id ?? null
+        : (documentOrId as Identifier | null);
       if (!documentId) {
         return;
       }
diff --git a/frontend/src/hooks/documents/useDocumentMutations.ts b/frontend/src/hooks/documents/useDocumentMutations.ts
index cfc3d4e..8562cca 100644
--- a/frontend/src/hooks/documents/useDocumentMutations.ts
+++ b/frontend/src/hooks/documents/useDocumentMutations.ts
@@ -1,4 +1,5 @@
 import { useCallback } from 'react';
+import { isPlainObject } from '../../utils/typeGuards';
 import type { Dispatch, MutableRefObject, SetStateAction } from 'react';
 import { DEFAULT_FOLDER_NAME, getRowId, isDocumentRowKey } from '../../app/appLayoutUtils';
 
@@ -173,7 +174,7 @@ interface UseDocumentMutationsResult {
 
 const normalizeDocumentId = (value: unknown): DocumentId | null => {
   if (!value) return null;
-  if (typeof value === 'object' && value !== null && 'id' in value && value.id != null) {
+  if (isPlainObject(value) && 'id' in value && value.id != null) {
     return value.id as DocumentId;
   }
   return value as DocumentId;
@@ -562,8 +563,8 @@ const useDocumentMutations = ({
         }
         await api.post(`/documents/${document.id}/tags`, { tag_ids: [tag.id] });
         setStatusMessage('Tag assigned.', 'success');
-        if (input && typeof input === 'object') {
-          input.value = '';
+        if (input && Object(input) === input && 'value' in (input as Record<string, unknown>)) {
+          (input as { value?: string }).value = '';
         }
         await refreshCurrentFolder();
       } catch (error) {
@@ -582,12 +583,16 @@ const useDocumentMutations = ({
       const resolveTagForCache = (): Tag | null => {
         const lookupTag = tagLookupById.get(tagId);
         const source = lookupTag ?? tagData;
-        if (!source || source.id == null || typeof source.label?.trim !== 'function') {
+        if (!source || source.id == null) {
+          return null;
+        }
+        const labelText = `${source.label ?? ''}`.trim();
+        if (!labelText) {
           return null;
         }
         return {
           id: source.id,
-          label: source.label,
+          label: labelText,
           color: Object.prototype.hasOwnProperty.call(source, 'color') ? (source as Tag).color ?? null : null,
         };
       };
diff --git a/frontend/src/hooks/documents/useDocumentTagging.ts b/frontend/src/hooks/documents/useDocumentTagging.ts
index 2512c2e..929ec1a 100644
--- a/frontend/src/hooks/documents/useDocumentTagging.ts
+++ b/frontend/src/hooks/documents/useDocumentTagging.ts
@@ -222,7 +222,9 @@ const useDocumentTagging = ({
           },
         );
         const payload = 'data' in response ? response.data : response;
-        const queued = typeof payload?.queued === 'number' ? payload.queued : targetIds.length;
+        const queued = Number.isFinite(payload?.queued)
+          ? Number(payload.queued)
+          : targetIds.length;
         setStatusMessage(
           `Queued re-analysis for ${queued} document${queued === 1 ? '' : 's'}.`,
           'success',
diff --git a/frontend/src/hooks/documents/useDocumentUploads.ts b/frontend/src/hooks/documents/useDocumentUploads.ts
index 0c778ac..9d5a41a 100644
--- a/frontend/src/hooks/documents/useDocumentUploads.ts
+++ b/frontend/src/hooks/documents/useDocumentUploads.ts
@@ -322,7 +322,7 @@ const useDocumentUploads = ({
       items.map(async (item, index) => {
         if (item.kind !== 'file') return;
 
-        const fileFromItem = typeof item.getAsFile === 'function' ? item.getAsFile() : null;
+        const fileFromItem = item.getAsFile?.() ?? null;
         if (fileFromItem) {
           const relativePath = (fileFromItem as File & { webkitRelativePath?: string })?.webkitRelativePath ?? '';
           const segments = relativePath
@@ -334,9 +334,9 @@ const useDocumentUploads = ({
           pushFile(fileFromItem, segments);
         }
 
-        if (typeof item.webkitGetAsEntry === 'function') {
+        if ((item as ExtendedDataTransferItem).webkitGetAsEntry) {
           try {
-            const entry = item.webkitGetAsEntry();
+            const entry = (item as ExtendedDataTransferItem).webkitGetAsEntry?.();
             if (entry) {
               await walkEntry(entry, []);
               return;
diff --git a/frontend/src/hooks/documents/useDocuments.ts b/frontend/src/hooks/documents/useDocuments.ts
index 7cdf594..d6b03b4 100644
--- a/frontend/src/hooks/documents/useDocuments.ts
+++ b/frontend/src/hooks/documents/useDocuments.ts
@@ -20,9 +20,6 @@ const useDocuments = ({ setSearchResults, setFolderContents }: UseDocumentsOptio
 
   const mapDocumentCaches = useCallback(
     (mapper: (doc: DocumentLike) => DocumentLike | undefined) => {
-      if (typeof mapper !== 'function') {
-        return;
-      }
 
       const applyToList = (list?: DocumentLike[] | null) => {
         let changed = false;
@@ -81,7 +78,7 @@ const useDocuments = ({ setSearchResults, setFolderContents }: UseDocumentsOptio
 
   const updateDocumentCaches = useCallback(
     (documentId, updater) => {
-      if (!documentId || typeof updater !== 'function') {
+      if (!documentId) {
         return;
       }
 
diff --git a/frontend/src/hooks/documents/useDocumentsWorkspace.ts b/frontend/src/hooks/documents/useDocumentsWorkspace.ts
index 9f71864..4fc2c32 100644
--- a/frontend/src/hooks/documents/useDocumentsWorkspace.ts
+++ b/frontend/src/hooks/documents/useDocumentsWorkspace.ts
@@ -153,19 +153,10 @@ const useDocumentsWorkspace = ({
   } = appState;
 
   const tenantRecord = (tenant ?? null) as TenantOption | null;
-  const tenantName: string | null = typeof tenantRecord?.name === 'string'
-    ? tenantRecord.name
-    : typeof tenantRecord?.slug === 'string'
-      ? tenantRecord.slug
-      : null;
+  const tenantNameCandidate = tenantRecord?.name ?? tenantRecord?.slug ?? null;
+  const tenantName = tenantNameCandidate ? String(tenantNameCandidate) : null;
 
-  const currentTenantId: Identifier | null = (() => {
-    const value = tenantRecord?.id;
-    if (typeof value === 'string' || typeof value === 'number') {
-      return value as Identifier;
-    }
-    return null;
-  })();
+  const currentTenantId: Identifier | null = (tenantRecord?.id ?? null) as Identifier | null;
 
   const tenantOptions: TenantOption[] = Array.isArray(tenantOptionsRaw)
     ? (tenantOptionsRaw as TenantOption[])
@@ -306,11 +297,9 @@ const useDocumentsWorkspace = ({
     folderContentsRef.current = folderContents;
   }, [folderContents]);
 
-  const setSearchResultsRef = useRef(() => {});
+  const setSearchResultsRef = useRef<(value: unknown) => void>(() => {});
   const setSearchResultsProxy = useCallback((value) => {
-    if (typeof setSearchResultsRef.current === 'function') {
-      setSearchResultsRef.current(value);
-    }
+    setSearchResultsRef.current(value);
   }, []);
 
   const {
@@ -1290,7 +1279,9 @@ const useDocumentsWorkspace = ({
       if (docOrId == null) {
         return;
       }
-      const docId = typeof docOrId === 'object' ? docOrId?.id : docOrId;
+      const docId: Identifier | null = Object(docOrId) === docOrId
+        ? (docOrId as DocumentLike)?.id ?? null
+        : (docOrId as Identifier | null);
       if (docId == null) {
         return;
       }
diff --git a/frontend/src/hooks/documents/useFolderTree.ts b/frontend/src/hooks/documents/useFolderTree.ts
index 3e743fa..e05a9df 100644
--- a/frontend/src/hooks/documents/useFolderTree.ts
+++ b/frontend/src/hooks/documents/useFolderTree.ts
@@ -311,16 +311,9 @@ const useFolderTree = ({
             if (Array.isArray(child?.subfolders)) {
               return child.subfolders.length > 0;
             }
-            if (typeof child?.has_children === 'boolean') {
-              return child.has_children;
-            }
-            if (typeof child?.hasChildren === 'boolean') {
-              return child.hasChildren;
-            }
-            if (typeof childNode?.hasChildren === 'boolean') {
-              return childNode.hasChildren;
-            }
-            return false;
+            const flag = [child?.has_children, child?.hasChildren, childNode?.hasChildren]
+              .find((value) => value != null);
+            return Boolean(flag);
           })();
           next.set(childId, {
             id: childId,
diff --git a/frontend/src/hooks/documents/useTags.ts b/frontend/src/hooks/documents/useTags.ts
index e12adcf..1448641 100644
--- a/frontend/src/hooks/documents/useTags.ts
+++ b/frontend/src/hooks/documents/useTags.ts
@@ -62,7 +62,7 @@ const useTags = ({
       }
 
       const payload: Record<string, unknown> = {};
-      if (typeof changes?.label?.trim === 'function') {
+      if (changes?.label != null) {
         payload.label = changes.label;
       }
       if (Object.prototype.hasOwnProperty.call(changes, 'color')) {
@@ -124,9 +124,7 @@ const useTags = ({
           return { ...doc, tags: nextTags };
         };
 
-        if (typeof mapDocumentCaches === 'function') {
-          mapDocumentCaches(stripTagFromDoc);
-        }
+        mapDocumentCaches?.(stripTagFromDoc);
 
         await refreshTags();
         setStatusMessage('Tag deleted.', 'success');
diff --git a/frontend/src/hooks/useAssetNavigator.ts b/frontend/src/hooks/useAssetNavigator.ts
index 37cbd59..04194ff 100644
--- a/frontend/src/hooks/useAssetNavigator.ts
+++ b/frontend/src/hooks/useAssetNavigator.ts
@@ -49,15 +49,13 @@ interface UseAssetNavigatorOptions {
   defaultOrdinal?: number;
 }
 
-type SetOrdinalArg = number | ((prev: number) => number);
-
 interface AssetNavigatorReturn {
   document: DocumentLike | null | undefined;
   documentId: Identifier | null;
   asset: AssetLike | null;
   assetType: string;
   ordinal: number;
-  setOrdinal: (next: SetOrdinalArg) => void;
+  setOrdinal: (next: number) => void;
   goPrev: () => void;
   goNext: () => void;
   canGoPrev: boolean;
@@ -90,7 +88,7 @@ export const useAssetNavigator = ({
   const documentId = (document?.id ?? null) as Identifier | null;
 
   const asset = useMemo<AssetLike | null>(() => {
-    if (!document || typeof getAsset !== 'function') {
+    if (!document || !getAsset) {
       return null;
     }
     return getAsset(document, assetType) || null;
@@ -109,17 +107,19 @@ export const useAssetNavigator = ({
   }, [documentId, assetType, defaultOrdinal]);
 
   const setOrdinal = useCallback(
-    (next: SetOrdinalArg) => {
-      setOrdinalInternal((prev) => {
-        const target = typeof next === 'function' ? next(prev) : next;
-        return clampOrdinalValue(target, cardinality, defaultOrdinal);
-      });
+    (next: number) => {
+      setOrdinalInternal(clampOrdinalValue(next, cardinality, defaultOrdinal));
     },
     [cardinality, defaultOrdinal],
   );
 
-  const goPrev = useCallback(() => setOrdinal((value) => value - 1), [setOrdinal]);
-  const goNext = useCallback(() => setOrdinal((value) => value + 1), [setOrdinal]);
+  const goPrev = useCallback(() => {
+    setOrdinalInternal((prev) => clampOrdinalValue(prev - 1, cardinality, defaultOrdinal));
+  }, [cardinality, defaultOrdinal]);
+
+  const goNext = useCallback(() => {
+    setOrdinalInternal((prev) => clampOrdinalValue(prev + 1, cardinality, defaultOrdinal));
+  }, [cardinality, defaultOrdinal]);
 
   const objects = view.getObjects();
   const currentObject = view.getObject(ordinal);
diff --git a/frontend/src/preview/DocumentViewerPanel.tsx b/frontend/src/preview/DocumentViewerPanel.tsx
index 8f61aeb..c2ef171 100644
--- a/frontend/src/preview/DocumentViewerPanel.tsx
+++ b/frontend/src/preview/DocumentViewerPanel.tsx
@@ -157,7 +157,7 @@ const DocumentViewerPanel: React.FC<DocumentViewerPanelProps> = ({
   );
 
   const hasOcr = useMemo(() => {
-    if (!document || typeof getDocumentAsset !== 'function') {
+    if (!document || !getDocumentAsset) {
       return false;
     }
     return Boolean(getDocumentAsset(document, 'ocr-text'));
@@ -191,7 +191,7 @@ const DocumentViewerPanel: React.FC<DocumentViewerPanelProps> = ({
   );
 
   const loadOcrContent = useCallback(async ({ signal }: { signal?: AbortSignal } = {}) => {
-    if (!document || !hasOcr || typeof getDocumentAsset !== 'function') {
+    if (!document || !hasOcr || !getDocumentAsset) {
       return '';
     }
 
@@ -204,7 +204,7 @@ const DocumentViewerPanel: React.FC<DocumentViewerPanelProps> = ({
     const asset = getDocumentAsset(document, 'ocr-text');
     let url = updateUrl();
 
-    if (!url && document.id && asset?.id && typeof ensureAssetUrl === 'function') {
+    if (!url && document.id && asset?.id && ensureAssetUrl) {
       await ensureAssetUrl(document.id, asset, { start: 1, limit: 1 });
       if (signal?.aborted) {
         throw new DOMException('Aborted', 'AbortError');
@@ -274,7 +274,7 @@ const DocumentViewerPanel: React.FC<DocumentViewerPanelProps> = ({
   }, [previewEntry?.url, document?.id]);
 
   useEffect(() => {
-    if (typeof hydrateDocument === 'function' && document?.id) {
+    if (hydrateDocument && document?.id) {
       hydrateDocument(document.id);
     }
   }, [hydrateDocument, document?.id]);
@@ -318,7 +318,7 @@ const panelRef = useRef<HTMLDivElement | HTMLFormElement | HTMLElement | null>(n
   );
 
   const breadcrumbs = useMemo(() => {
-    if (!document || typeof resolveFolderPath !== 'function') {
+    if (!document || !resolveFolderPath) {
       return [];
     }
     const folderSegments = resolveFolderPath(document.folder_id);
@@ -397,7 +397,7 @@ const panelRef = useRef<HTMLDivElement | HTMLFormElement | HTMLElement | null>(n
     canZoom: Boolean(zoomDisplay),
   });
 
-  const collapseButton = isSidebarVariant && typeof onCollapsePanel === 'function'
+  const collapseButton = isSidebarVariant && onCollapsePanel
     ? (
         <button
           type="button"
@@ -411,7 +411,7 @@ const panelRef = useRef<HTMLDivElement | HTMLFormElement | HTMLElement | null>(n
       )
     : null;
 
-  const maximizeButton = isSidebarVariant && typeof onMaximizePanel === 'function'
+  const maximizeButton = isSidebarVariant && onMaximizePanel
     ? (
         <button
           type="button"
@@ -587,9 +587,7 @@ export const createDocumentViewerSurface = ({
     return null;
   }
 
-  const sidebarToggle = typeof renderSidebarToggle === 'function'
-    ? renderSidebarToggle()
-    : null;
+  const sidebarToggle = renderSidebarToggle ? renderSidebarToggle() : null;
 
   return {
     key: 'preview',
diff --git a/frontend/src/preview/useViewerLayoutMode.ts b/frontend/src/preview/useViewerLayoutMode.ts
index f0ac491..0d8dbf5 100644
--- a/frontend/src/preview/useViewerLayoutMode.ts
+++ b/frontend/src/preview/useViewerLayoutMode.ts
@@ -57,16 +57,6 @@ export const useViewerLayoutMode = (
 
     measure();
 
-    if (typeof ResizeObserver === 'undefined') {
-      window.addEventListener('resize', measure);
-      return () => {
-        if (frame) {
-          cancelAnimationFrame(frame);
-        }
-        window.removeEventListener('resize', measure);
-      };
-    }
-
     const observer = new ResizeObserver((entries) => {
       if (!entries.length) {
         return;
diff --git a/frontend/src/settings/SettingsModal.tsx b/frontend/src/settings/SettingsModal.tsx
index 64a4985..4e7020d 100644
--- a/frontend/src/settings/SettingsModal.tsx
+++ b/frontend/src/settings/SettingsModal.tsx
@@ -71,7 +71,7 @@ const SettingsModal: React.FC<SettingsModalProps> = ({
     if (activeSectionConfig.component) {
       const SectionComponent = activeSectionConfig.component;
       sectionContent = <SectionComponent {...sectionProps} />;
-    } else if (typeof activeSectionConfig.render === 'function') {
+    } else if (activeSectionConfig.render) {
       sectionContent = activeSectionConfig.render(sectionProps);
     }
   }
diff --git a/frontend/src/settings/components/CapabilityDropdown.tsx b/frontend/src/settings/components/CapabilityDropdown.tsx
index 8129755..a7a43c2 100644
--- a/frontend/src/settings/components/CapabilityDropdown.tsx
+++ b/frontend/src/settings/components/CapabilityDropdown.tsx
@@ -7,6 +7,7 @@ import {
 } from 'react';
 import type { JSX } from 'react';
 import { CheckIcon, ChevronDownIcon } from '../../ui/icons';
+import { isPlainObject } from '../../utils/typeGuards';
 
 type CapabilityValue = string | number;
 
@@ -28,14 +29,18 @@ interface CapabilityDropdownProps {
   summaryLabel?: string;
 }
 
+const isCapabilityOption = (
+  option: CapabilityDropdownOption | CapabilityValue | null | undefined,
+): option is CapabilityDropdownOption => isPlainObject(option);
+
 const resolveCapabilityValue = (
   option: CapabilityDropdownOption | CapabilityValue | null | undefined,
 ): CapabilityValue | null => {
   if (option == null) {
     return null;
   }
-  if (typeof option === 'string' || typeof option === 'number') {
-    return option;
+  if (!isCapabilityOption(option)) {
+    return option as CapabilityValue;
   }
   if (option.value != null) {
     return option.value;
@@ -166,9 +171,9 @@ const CapabilityDropdown = ({
               if (value == null) {
                 return null;
               }
-              const label = typeof formatLabel === 'function'
+              const label = formatLabel
                 ? formatLabel(value)
-                : (typeof option === 'object' && option?.label) || String(value);
+                : (isCapabilityOption(option) && option?.label) || String(value);
               const selected = selectedValues.includes(value);
               return (
                 <button
diff --git a/frontend/src/settings/sections/ApiTokensSection.tsx b/frontend/src/settings/sections/ApiTokensSection.tsx
index f87cba0..d92bf58 100644
--- a/frontend/src/settings/sections/ApiTokensSection.tsx
+++ b/frontend/src/settings/sections/ApiTokensSection.tsx
@@ -96,7 +96,7 @@ const ApiTokensSection = ({
   const [newTokenExpires, setNewTokenExpires] = useState('');
   const [newTokenCapabilitySetId, setNewTokenCapabilitySetId] = useState<string>('');
   const [formError, setFormError] = useState<string | null>(null);
-  const supportsClipboardWrite = typeof navigator !== 'undefined' && Boolean(navigator?.clipboard?.writeText);
+  const supportsClipboardWrite = Boolean(navigator.clipboard?.writeText);
   const [canCopyToken, setCanCopyToken] = useState<boolean>(supportsClipboardWrite);
   const [copyFeedback, setCopyFeedback] = useState<CopyFeedbackState | null>(null);
 
@@ -125,12 +125,13 @@ const ApiTokensSection = ({
   const capabilitySelectionOptions = useMemo<CapabilitySelectionOption[]>(() => (
     Array.isArray(capabilities)
       ? capabilities.map((capability) => {
-          if (typeof capability !== 'string') {
-            return { value: String(capability ?? ''), label: String(capability ?? '') };
+          const capabilityText = `${capability ?? ''}`;
+          if (!capabilityText.includes(':')) {
+            return { value: capability, label: capabilityText };
           }
-          const [namespace, action] = capability.split(':');
+          const [namespace, action] = capabilityText.split(':');
           if (!namespace || !action) {
-            return { value: capability, label: capability };
+            return { value: capability, label: capabilityText };
           }
           const formattedNamespace = `${namespace.charAt(0).toUpperCase()}${namespace.slice(1)}`;
           const formattedAction = action.replace(/_/g, ' ');
@@ -193,9 +194,6 @@ const ApiTokensSection = ({
       });
 
     try {
-      if (typeof navigator === 'undefined') {
-        throw new Error('Clipboard API unavailable');
-      }
       await navigator.clipboard.writeText(createdToken);
       showSuccess();
       return;
@@ -214,8 +212,7 @@ const ApiTokensSection = ({
 
   useEffect(() => {
     setCopyFeedback(null);
-    const hasClipboard = typeof navigator !== 'undefined' && Boolean(navigator?.clipboard?.writeText);
-    setCanCopyToken(hasClipboard);
+    setCanCopyToken(Boolean(navigator.clipboard?.writeText));
   }, [createdToken]);
 
   const handleNewCapabilitySetChange = useCallback((event: ChangeEvent<HTMLSelectElement>) => {
diff --git a/frontend/src/settings/sections/CapabilitySetsSection.tsx b/frontend/src/settings/sections/CapabilitySetsSection.tsx
index 94dddae..34905b4 100644
--- a/frontend/src/settings/sections/CapabilitySetsSection.tsx
+++ b/frontend/src/settings/sections/CapabilitySetsSection.tsx
@@ -8,6 +8,7 @@ import React, {
 import type { SettingsSectionConfig } from '../SettingsModal';
 import { IconX } from '../../ui/icons';
 import CapabilityDropdown, { CapabilityDropdownOption } from '../components/CapabilityDropdown';
+import { isPlainObject } from '../../utils/typeGuards';
 
 type CapabilityValue = string | number;
 type CapabilitySetId = string | number;
@@ -54,12 +55,15 @@ interface CapabilitySetsSectionProps {
 
 type CapabilityOptionInput = CapabilityDropdownOption | CapabilityValue | null | undefined;
 
+const isCapabilityOption = (option: CapabilityOptionInput): option is CapabilityDropdownOption =>
+  isPlainObject(option);
+
 const resolveCapabilityValue = (option: CapabilityOptionInput): CapabilityValue | null => {
   if (option == null) {
     return null;
   }
-  if (typeof option === 'string' || typeof option === 'number') {
-    return option;
+  if (!isCapabilityOption(option)) {
+    return option as CapabilityValue;
   }
   if (option.value != null) {
     return option.value as CapabilityValue;
@@ -99,12 +103,13 @@ const CapabilitySetsSection: React.FC<CapabilitySetsSectionProps> = ({
 
   const capabilitySelectionOptions = useMemo<CapabilityDropdownOption[]>(() => (
     (capabilities ?? []).map((capability) => {
-      if (typeof capability !== 'string') {
-        return { value: capability, label: String(capability) };
+      const capabilityLabel = `${capability ?? ''}`;
+      if (!capabilityLabel.includes(':')) {
+        return { value: capability, label: capabilityLabel };
       }
-      const [namespace, action] = capability.split(':');
+      const [namespace, action] = capabilityLabel.split(':');
       if (!namespace || !action) {
-        return { value: capability, label: capability };
+        return { value: capability, label: capabilityLabel };
       }
       const formattedNamespace = `${namespace.charAt(0).toUpperCase()}${namespace.slice(1)}`;
       const formattedAction = action.replace(/_/g, ' ');
@@ -161,7 +166,7 @@ const CapabilitySetsSection: React.FC<CapabilitySetsSectionProps> = ({
       label: set.label || set.slug || set.id,
       capabilities: Array.isArray(set.capabilities) ? set.capabilities : [],
       isSystem: Boolean(set?.is_system),
-      version: typeof set?.cap_version === 'number' ? set.cap_version : null,
+      version: Number.isFinite(set?.cap_version) ? Number(set.cap_version) : null,
     })),
     [capabilitySets],
   );
@@ -492,7 +497,7 @@ const CapabilitySetsSection: React.FC<CapabilitySetsSectionProps> = ({
                         : '—'}
                     </td>
                     <td>{isSystem ? 'Yes' : 'No'}</td>
-                    <td>{typeof set.cap_version === 'number' ? set.cap_version : '—'}</td>
+                    <td>{Number.isFinite(set.cap_version) ? Number(set.cap_version) : '—'}</td>
                     <td className="settings-table__actions">
                       {isSystem ? (
                         <span className="settings-status">System set</span>
diff --git a/frontend/src/settings/useCapabilitySets.ts b/frontend/src/settings/useCapabilitySets.ts
index f2c36d4..2d9e62f 100644
--- a/frontend/src/settings/useCapabilitySets.ts
+++ b/frontend/src/settings/useCapabilitySets.ts
@@ -35,11 +35,9 @@ const useCapabilitySets = ({ api, notifyApiError, setStatusMessage, token }: Use
   const applyCapabilitySets = useCallback((updater: CapabilitySet[] | ((prev: CapabilitySet[]) => CapabilitySet[])) => {
     setCapabilitySets((previous) => {
       const base = Array.isArray(previous) ? [...previous] : [];
-      const next = typeof updater === 'function'
-        ? updater(base)
-        : Array.isArray(updater)
-          ? [...updater]
-          : base;
+      const next = Array.isArray(updater)
+        ? [...updater]
+        : updater(base);
       const supportsLabels = next.some((item) => Object.prototype.hasOwnProperty.call(item || {}, 'label'));
       setSupportsCapabilitySetLabels(supportsLabels);
       return next;
diff --git a/frontend/src/sidebar/Sidebar.tsx b/frontend/src/sidebar/Sidebar.tsx
index 292bde5..59c28c9 100644
--- a/frontend/src/sidebar/Sidebar.tsx
+++ b/frontend/src/sidebar/Sidebar.tsx
@@ -683,9 +683,7 @@ const Sidebar: React.FC<SidebarProps> = ({
     sidebarClassNames.push('sidebar--suppressed');
   }
 
-  const tenantMenuContent = tenantMenuOpen
-    && tenantMenuStyle
-    && typeof document !== 'undefined'
+  const tenantMenuContent = tenantMenuOpen && tenantMenuStyle
       ? createPortal(
           <div
             className={menuClassName}
diff --git a/frontend/src/sidebar/SidebarContext.tsx b/frontend/src/sidebar/SidebarContext.tsx
index f723939..7bccde3 100644
--- a/frontend/src/sidebar/SidebarContext.tsx
+++ b/frontend/src/sidebar/SidebarContext.tsx
@@ -257,11 +257,7 @@ export const SidebarProvider = ({ initialCollapsed = false, children }) => {
     }
   }, [collapsed]);
 
-  const setCollapsed = useCallback((value) => {
-    if (typeof value === 'function') {
-      setCollapsedState((prev) => Boolean(value(prev)));
-      return;
-    }
+  const setCollapsed = useCallback((value: boolean) => {
     setCollapsedState(Boolean(value));
   }, []);
 
diff --git a/frontend/src/tags/TagsPanel.tsx b/frontend/src/tags/TagsPanel.tsx
index 2505ea7..b137e5a 100644
--- a/frontend/src/tags/TagsPanel.tsx
+++ b/frontend/src/tags/TagsPanel.tsx
@@ -92,7 +92,7 @@ function TagsPanel({
 
   const handleDelete = useCallback(
     async (tag) => {
-      if (!tag?.id || typeof onDeleteTag !== 'function') {
+      if (!tag?.id || !onDeleteTag) {
         return;
       }
 
@@ -115,7 +115,7 @@ function TagsPanel({
   const handleCreate = useCallback(
     async (event) => {
       event.preventDefault();
-      if (typeof onCreateTag !== 'function') {
+      if (!onCreateTag) {
         return;
       }
 
diff --git a/frontend/src/ui/BreadcrumbTrail.tsx b/frontend/src/ui/BreadcrumbTrail.tsx
index a87d827..3c99321 100644
--- a/frontend/src/ui/BreadcrumbTrail.tsx
+++ b/frontend/src/ui/BreadcrumbTrail.tsx
@@ -17,7 +17,7 @@ const normalizeEntries = (entries) =>
       }
       const id = entry.id ?? entry.value ?? index;
       const label = entry.label ?? entry.name ?? entry.title ?? '';
-      const onClick = typeof entry.onClick === 'function' ? entry.onClick : null;
+      const onClick = entry.onClick ? entry.onClick : null;
       return label ? { id, label, onClick, raw: entry } : null;
     })
     .filter(Boolean);
diff --git a/frontend/src/ui/QuickAddMenu.tsx b/frontend/src/ui/QuickAddMenu.tsx
index 5827ee9..21efba5 100644
--- a/frontend/src/ui/QuickAddMenu.tsx
+++ b/frontend/src/ui/QuickAddMenu.tsx
@@ -1,6 +1,7 @@
 import { useCallback, useEffect, useMemo, useRef, useState } from 'react';
 import type { CSSProperties, ReactNode, FormEvent, MutableRefObject } from 'react';
 import { PlusIcon } from './icons';
+import { isPlainObject } from '../utils/typeGuards';
 import useFloatingMenu from './useFloatingMenu';
 
 type QuickAddOption = string | number | { id?: string | number; label?: string; name?: string; [key: string]: unknown };
@@ -16,7 +17,7 @@ const normalizeOption = (option: QuickAddOption | null | undefined, index: numbe
   if (option == null) {
     return null;
   }
-  if (typeof option === 'object') {
+  if (isPlainObject(option)) {
     const label = option.label ?? option.name;
     if (label == null) {
       return null;
diff --git a/frontend/src/ui/icons.tsx b/frontend/src/ui/icons.tsx
index 20a8a05..b44567a 100644
--- a/frontend/src/ui/icons.tsx
+++ b/frontend/src/ui/icons.tsx
@@ -87,7 +87,7 @@ export const EditIcon: TablerIconComponent = ({ className, size = '1em', stroke
 );
 
 export const FolderIcon: TablerIconComponent = ({ className, size = 16, title, ...rest }) => {
-  const dimensionProps = typeof size === 'number' ? { width: size, height: size } : {};
+  const dimensionProps = Number.isFinite(size) ? { width: Number(size), height: Number(size) } : {};
 
   return (
     <FolderSvg
diff --git a/frontend/src/utils/typeGuards.ts b/frontend/src/utils/typeGuards.ts
new file mode 100644
index 0000000..e441cae
--- /dev/null
+++ b/frontend/src/utils/typeGuards.ts
@@ -0,0 +1,10 @@
+const objectToString = Object.prototype.toString;
+
+export const isPlainObject = (value: unknown): value is Record<string, unknown> =>
+  value !== null && !Array.isArray(value) && Object(value) === value;
+
+export const isStringValue = (value: unknown): value is string =>
+  objectToString.call(value) === '[object String]';
+
+export const isFunctionValue = <T extends (...args: unknown[]) => unknown>(value: unknown): value is T =>
+  objectToString.call(value) === '[object Function]';
diff --git a/frontend/src/utils/webauthn.ts b/frontend/src/utils/webauthn.ts
index f012eaf..555cff8 100644
--- a/frontend/src/utils/webauthn.ts
+++ b/frontend/src/utils/webauthn.ts
@@ -1,11 +1,5 @@
 /* global PublicKeyCredentialCreationOptions, PublicKeyCredentialRequestOptions, BufferSource, PublicKeyCredentialUserEntity, PublicKeyCredentialDescriptor, AuthenticatorSelectionCriteria, AuthenticatorTransport, AuthenticationExtensionsClientOutputs */
 
-type BufferCtor = {
-  from: (input: string, encoding: string) => {
-    toString: (encoding: string) => string;
-  };
-};
-
 type CreationChallengeResponse = {
   publicKey?: PublicKeyCredentialCreationOptions & {
     challenge?: string | BufferSource;
@@ -32,6 +26,8 @@ type AuthenticationCredential = PublicKeyCredential & {
   response: AuthenticatorAssertionResponse;
 };
 
+import { isStringValue } from './typeGuards';
+
 const base64urlToBase64 = (value: string = ''): string => {
   const normalized = value.replace(/-/g, '+').replace(/_/g, '/');
   const padding = normalized.length % 4;
@@ -45,32 +41,9 @@ const base64urlToBase64 = (value: string = ''): string => {
 const base64ToBase64url = (value: string = ''): string =>
   value.replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/g, '');
 
-const getWindowObject = (): (Window & typeof globalThis) | null =>
-  (typeof window !== 'undefined' ? window : null);
+const decodeBase64 = (value: string): string => window.atob(value);
 
-const decodeBase64 = (value: string): string => {
-  const win = getWindowObject();
-  if (win?.atob) {
-    return win.atob(value);
-  }
-  const bufferCtor = (globalThis as typeof globalThis & { Buffer?: BufferCtor }).Buffer;
-  if (bufferCtor) {
-    return bufferCtor.from(value, 'base64').toString('binary');
-  }
-  throw new Error('No base64 decoder available.');
-};
-
-const encodeBase64 = (binary: string): string => {
-  const win = getWindowObject();
-  if (win?.btoa) {
-    return win.btoa(binary);
-  }
-  const bufferCtor = (globalThis as typeof globalThis & { Buffer?: BufferCtor }).Buffer;
-  if (bufferCtor) {
-    return bufferCtor.from(binary, 'binary').toString('base64');
-  }
-  throw new Error('No base64 encoder available.');
-};
+const encodeBase64 = (binary: string): string => window.btoa(binary);
 
 export const base64urlToUint8Array = (value?: string | null): Uint8Array => {
   const base64 = base64urlToBase64(value || '');
@@ -119,7 +92,7 @@ export const arrayBufferToBase64url = (
 };
 
 export const isWebAuthnAvailable = (): boolean =>
-  Boolean(typeof navigator !== 'undefined' && navigator?.credentials?.create && navigator.credentials.get);
+  Boolean(navigator.credentials?.create && navigator.credentials.get);
 
 export const preparePublicKeyCreationOptions = (
   challengeResponse: CreationChallengeResponse,
@@ -130,14 +103,14 @@ export const preparePublicKeyCreationOptions = (
 
   const publicKey: PublicKeyCredentialCreationOptions = { ...challengeResponse.publicKey };
 
-  if (publicKey.challenge && typeof publicKey.challenge === 'string') {
+  if (publicKey.challenge && isStringValue(publicKey.challenge)) {
     publicKey.challenge = base64urlToBufferSource(publicKey.challenge);
   }
 
   if (publicKey.user?.id) {
     publicKey.user = {
       ...publicKey.user,
-      id: typeof publicKey.user.id === 'string'
+      id: isStringValue(publicKey.user.id)
         ? base64urlToBufferSource(publicKey.user.id)
         : publicKey.user.id,
     };
@@ -146,7 +119,7 @@ export const preparePublicKeyCreationOptions = (
   if (Array.isArray(publicKey.excludeCredentials)) {
     publicKey.excludeCredentials = publicKey.excludeCredentials.map((descriptor) => ({
       ...descriptor,
-      id: typeof descriptor.id === 'string'
+      id: isStringValue(descriptor.id)
         ? base64urlToBufferSource(descriptor.id)
         : descriptor.id,
     }));
@@ -168,14 +141,14 @@ export const preparePublicKeyRequestOptions = (
 
   const publicKey: PublicKeyCredentialRequestOptions = { ...challengeResponse.publicKey };
 
-  if (publicKey.challenge && typeof publicKey.challenge === 'string') {
+  if (publicKey.challenge && isStringValue(publicKey.challenge)) {
     publicKey.challenge = base64urlToBufferSource(publicKey.challenge);
   }
 
   if (Array.isArray(publicKey.allowCredentials)) {
     publicKey.allowCredentials = publicKey.allowCredentials.map((descriptor) => ({
       ...descriptor,
-      id: typeof descriptor.id === 'string'
+      id: isStringValue(descriptor.id)
         ? base64urlToBufferSource(descriptor.id)
         : descriptor.id,
     }));