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/shortcut-definitions.ts

@@ -102,12 +102,12 @@ async function _pasteInternal(ctx: ShortcutContext): Promise<void> {
   ctx.onChange();
 }
 
-/** Run op on selected items → update transform box → fire onChange. */
+/** Run op on selected items → update transform box → fire onChange with IDs. */
 function _opUpdate(ctx: ShortcutContext, op: (items: SceneItem[]) => void): void {
   const items = ctx.selection.getSelectedItems();
   op(items);
   ctx.selection.transformBox.update(items);
-  ctx.onChange();
+  ctx.onChange(items.map(i => i.id));
 }
 
 export const shortcuts: ShortcutDef[] = [