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/canvas/shortcuts.ts

@@ -31,7 +31,7 @@ export interface ShortcutContext {
   selection: SelectionManager;
   history: UndoManager;
   viewport: Viewport;
-  onChange: () => void;
+  onChange: (changedIds?: string[]) => void;
   clipboardRef: React.MutableRefObject<SceneItem[]>;
   showToast: (msg: string) => void;
   fitAll: () => void;