fix: hardening pass — permissions, socket reconnect, canvas setup, arrangements

- Fix 403 on save for public collection viewers (return role in GET board response)
- Add read-only status indicator (StatusBar + StatusIndicator)
- Fix beforeunload save to use fetch+keepalive with auth header
- Socket reconnect now rejoins board room automatically
- Canvas setup uses polling instead of brittle 200ms timer
- Fix double user:left on disconnect (use disconnecting event, snapshot rooms)
- Thread + comment creation wrapped in db.transaction
- Prevent owner downgrade via addCollectionMember (check existing member)
- Bound redirect depth in downloadImage to 5
- Arrangement operations anchor to bounding box top-left (no drift)
- Distribute H/V also anchor to top-left
- Fix annotations fetch to use axios api instance (401 interceptor)
- Replace require() with static import in shortcut-definitions

backend/routes/boards.js

@@ -153,6 +153,7 @@ router.get('/:boardId', (req, res) => {
         name: collection.name,
       },
       images,
+      role: result.member?.role || 'viewer',
     });
   } catch (err) {
     console.error('[boards] get error:', err);