refactor: introduce dedicated identifier types for improved clarity and type safety

frontend/src/correspondents/CorrespondentsPanel.tsx

@@ -7,7 +7,7 @@ interface CorrespondentUsage {
 }
 
 export interface CorrespondentEntry {
-  id?: string | number;
+  id?: string;
   name?: string;
   usage?: CorrespondentUsage;
   [key: string]: unknown;
@@ -17,8 +17,8 @@ export interface CorrespondentsPanelProps {
   correspondents?: CorrespondentEntry[];
   onRefresh?: () => void | Promise<void>;
   onCreate: (payload: { name: string }) => Promise<CorrespondentEntry | void>;
-  onUpdate: (id: string | number, payload: { name: string }) => Promise<void>;
-  onDelete: (id: string | number) => Promise<void>;
+  onUpdate: (id: string, payload: { name: string }) => Promise<void>;
+  onDelete: (id: string) => Promise<void>;
   onNotify?: (message: string, variant?: string) => void;
 }
 
@@ -30,12 +30,12 @@ function CorrespondentsPanel({
   onDelete,
   onNotify,
 }: CorrespondentsPanelProps) {
-  const [editingId, setEditingId] = useState<string | number | null>(null);
+  const [editingId, setEditingId] = useState<string | null>(null);
   const [draftName, setDraftName] = useState('');
   const [createName, setCreateName] = useState('');
   const [saving, setSaving] = useState(false);
   const [creating, setCreating] = useState(false);
-  const [deletingId, setDeletingId] = useState<string | number | null>(null);
+  const [deletingId, setDeletingId] = useState<string | null>(null);
 
   const startEdit = useCallback((correspondent: CorrespondentEntry) => {
     setEditingId(correspondent.id);