refactor: centralize multi-card bring-to-front logic and document new spatial workspace architecture.

frontend/src/desktop/LayoutSystem.ts

@@ -274,7 +274,6 @@ export class LayoutStore {
             const dx = card.x - x;
             const dy = card.y - y;
 
-            // Check if center is within inner radius of top card
             // Use squared distance to avoid sqrt
             const distSq = dx * dx + dy * dy;
             if (distSq < radius * radius) {
@@ -296,6 +295,20 @@ export class LayoutStore {
             .map(c => c.id);
     }
 
+    bringToFront(ids: string[]) {
+        const cards = ids
+            .map(id => this.items.get(id))
+            .filter((c): c is LayoutCard => !!c);
+
+        // Sort by current Z-index to preserve relative order
+        cards.sort((a, b) => a.z - b.z);
+
+        // Assign new Z-indices
+        for (const card of cards) {
+            card.update({ z: this.zCounter++ });
+        }
+    }
+
     getSnapshot() {
         return Array.from(this.items.values()).map(card => card.toSnapshot());
     }

frontend/src/desktop/useCardPointer.ts

@@ -51,16 +51,30 @@ export const useCardPointer = (
 
         if (state === 'drag-start') {
             let effectiveSelection = selection;
-            if (!isSelected) {
-                onSelect([card.id], hasModifier);
-                if (hasModifier) {
-                    effectiveSelection = [...selection, card.id];
-                } else {
+
+            if (!hasModifier) {
+                if (!isSelected) {
                     effectiveSelection = [card.id];
+                    onSelect([card.id], false);
+                }
+            } else {
+                // Modifier pressed: Add card and stack to selection
+                const stackIds = card.store.getStackBelow(card);
+                const idsToAdd = new Set<string>();
+
+                if (!isSelected) idsToAdd.add(card.id);
+                stackIds.forEach(id => idsToAdd.add(id));
+
+                // Only select what isn't already selected to avoid toggling off
+                const unselectedIdsToAdd = Array.from(idsToAdd).filter(id => !selection.includes(id));
+
+                if (unselectedIdsToAdd.length > 0) {
+                    onSelect(unselectedIdsToAdd, true);
+                    effectiveSelection = [...selection, ...unselectedIdsToAdd];
                 }
             }
 
-            card.bringToFront();
+            card.store.bringToFront(effectiveSelection);
 
             setState('drag');