fix(refboard): full sync audit — batch multi-drag, persist resize, sync lock

- Batch broadcastTransform: sends all selected items in one emit instead
  of throttle-dropping all but the first (multi-select drag now syncs all)
- TransformBox onDragEnd: persist+sync final state after resize/rotate
  (was only broadcasting ephemeral transforms, never the final position)
- Layer lock toggle: add missing onCanvasChange() call for sync+save
- Receiver handles both batched and legacy single-item transform formats

frontend/src/hooks/useCanvasSetup.ts

@@ -126,13 +126,20 @@ export function useCanvasSetup(deps: CanvasSetupDeps) {
           },
         });
 
-        // Wire live drag/resize transforms to sync broadcast
+        // Wire live drag/resize transforms to sync broadcast (batched for multi-select)
+        selection.onItemsTransform = (items) => {
+          syncRef.current?.broadcastTransform(items);
+        };
         selection.onItemTransform = (item) => {
           syncRef.current?.broadcastTransform(item);
         };
         selection.transformBox.onItemTransform = (item) => {
           syncRef.current?.broadcastTransform(item);
         };
+        selection.transformBox.onDragEnd = () => {
+          syncRef.current?.broadcastSceneNow();
+          onCanvasChange();
+        };
 
         socket.on('user:joined', (data: any) => {
           const uid = data.userId || data.id;