feat: board thumbnails, inline rename, UX overhaul + zoom/counter fixes

- Auto-generate board thumbnail from canvas content on save (webp, 400px)
- Collection cards show stacked/fanned board thumbnails with random offsets
- Three-dot menu on all cards with Rename, Share, Delete actions
- Inline rename for collections (header + card) and boards
- Collection cards show public/private status and member count
- Share button on collection cards for quick access
- Add updateCollection/updateBoard API functions
- Add thumbnail + object_count columns to boards table with migration
- Fix zoom drift: use element-relative coords (offsetX/Y) not getScenePoint
- Fix thumbnail generation breaking viewport: use finally block for restore
- Fix thumbnail bounds: use scene-space obj coords not screen-space getBoundingRect
- Fix object counter: live count from canvas instead of stale DB images table
- Fix three-dot menu clipping by removing overflow:hidden from card containers
- Status bar shows "objects" instead of "images", updates on add/delete

frontend/src/canvas/FabricCanvas.tsx

@@ -138,9 +138,9 @@ const FabricCanvas = forwardRef<FabricCanvasHandle, FabricCanvasProps>(
         zoom *= 1 - direction * ZOOM_STEP;
         zoom = Math.max(MIN_ZOOM, Math.min(MAX_ZOOM, zoom));
 
-        // Zoom toward cursor position (industry standard)
-        const point = canvas.getScenePoint(e);
-        canvas.zoomToPoint(point, zoom);
+        // Zoom toward cursor — must use element-relative coords (offsetX/Y),
+        // NOT getScenePoint() which returns scene-space and causes zoom drift
+        canvas.zoomToPoint({ x: e.offsetX, y: e.offsetY } as any, zoom);
         canvas.requestRenderAll();
         persistViewport();
       });