feat(refboard): incremental sync — stop full-scene broadcast on edits

Replace broadcastSceneNow() with incremental element:update for all
property-modifying operations (align, arrange, flip, resize, rotate,
normalize, frame color). Full scene sync now only fires via debounced
fallback (500ms) for structural changes (add/remove/paste/group).

Changes:
- onCanvasChange() accepts optional changedIds for incremental sync
- SelectionToolbar, context menu, shortcuts all pass item IDs
- TransformBox.onDragEnd passes item IDs instead of full scene
- ShortcutContext, MenuContext, hook interfaces updated

This dramatically reduces sync traffic during normal editing — only
the changed elements are sent instead of the entire board state.

frontend/src/hooks/useCanvasSetup.ts

@@ -40,7 +40,7 @@ interface CanvasSetupDeps {
   undoRef: React.MutableRefObject<UndoManager | null>;
   syncRef: React.MutableRefObject<SyncHandle | null>;
   inboxZoneRef: React.MutableRefObject<InboxZone | null>;
-  onCanvasChange: () => void;
+  onCanvasChange: (changedIds?: string[]) => void;
   showToast: (msg: string) => void;
   setOnlineUsers: React.Dispatch<React.SetStateAction<OnlineUser[]>>;
   setSelectedLayerIds: React.Dispatch<React.SetStateAction<string[]>>;
@@ -136,9 +136,9 @@ export function useCanvasSetup(deps: CanvasSetupDeps) {
         selection.transformBox.onItemTransform = (item) => {
           syncRef.current?.broadcastTransform(item);
         };
-        selection.transformBox.onDragEnd = () => {
-          syncRef.current?.broadcastSceneNow();
-          onCanvasChange();
+        selection.transformBox.onDragEnd = (itemIds) => {
+          syncRef.current?.broadcastElements(itemIds);
+          onCanvasChange(itemIds);
         };
 
         socket.on('user:joined', (data: any) => {