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

backend/routes/boards.js

@@ -169,12 +169,12 @@ router.post('/:boardId/save', (req, res) => {
     const result = resolveBoard(req, res, 'editor');
     if (!result) return;
 
-    const { canvas_state } = req.body;
+    const { canvas_state, thumbnail } = req.body;
     if (canvas_state === undefined) {
       return res.status(400).json({ error: 'canvas_state is required' });
     }
 
-    saveBoardCanvas(result.board.id, canvas_state);
+    saveBoardCanvas(result.board.id, canvas_state, thumbnail || null);
 
     return res.json({ message: 'Canvas saved' });
   } catch (err) {