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/components/StatusBar.tsx

@@ -56,7 +56,7 @@ export default function StatusBar({ boardName, imageCount, saveStatus }: StatusB
     <div style={styles.bar}>
       <div style={styles.left}>
         <span style={styles.name}>{boardName}</span>
-        <span>{imageCount} image{imageCount !== 1 ? 's' : ''}</span>
+        <span>{imageCount} object{imageCount !== 1 ? 's' : ''}</span>
       </div>
       <div style={styles.right}>
         <span style={{ ...styles.dot, background: status.color }} />