feat(markdown): add double-click → edit mode via SelectionManager callback

frontend/src/canvas/SelectionManager.ts

@@ -85,6 +85,7 @@ export class SelectionManager {
 
   private _onDoubleClickText: ((item: SceneItem) => void) | null = null;
   private _onDoubleClickImage: ((item: SceneItem) => void) | null = null;
+  private _onDoubleClickMarkdown: ((item: SceneItem) => void) | null = null;
 
   private _enabled = true;
 
@@ -168,6 +169,11 @@ export class SelectionManager {
     this._onDoubleClickImage = fn;
   }
 
+  /** Called on double-click of a markdown item (for edit mode). */
+  set onDoubleClickMarkdown(fn: (item: SceneItem) => void) {
+    this._onDoubleClickMarkdown = fn;
+  }
+
   /** Select only this item, deselecting everything else. */
   selectOnly(id: string): void {
     this.selectedIds.clear();
@@ -393,6 +399,8 @@ export class SelectionManager {
           this._onDoubleClickText?.(item);
         } else if (item.type === 'image' || item.type === 'drawing') {
           this._onDoubleClickImage?.(item);
+        } else if (item.type === 'markdown') {
+          this._onDoubleClickMarkdown?.(item);
         }
         this._lastClickTime = 0;
         this._lastClickItemId = null;

frontend/src/hooks/useCanvasSetup.ts

@@ -158,6 +158,14 @@ export function useCanvasSetup(deps: CanvasSetupDeps) {
         });
       };
 
+      // Double-click markdown → enter edit mode
+      selection.onDoubleClickMarkdown = (item) => {
+        if (mdOverlayRef.current) {
+          mdOverlayRef.current.setEditing(item.id);
+          selection.setEnabled(false);
+        }
+      };
+
       // Refresh transform box when item dimensions change (e.g. video metadata loaded)
       scene.onItemDimensionsChanged = (itemId: string) => {
         if (selection.selectedIds.has(itemId)) {