finish refactor pre merge

lib/editor/draft/useUndoStack.ts

@@ -8,6 +8,7 @@ type Options = {
 
 export function useUndoStack(options: Options) {
     const { applyUndoAction } = options;
+    // Stack thao tác undo (append-only, pop khi undo).
     const [undoStack, setUndoStack] = useState<UndoAction[]>([]);
 
     const pushUndo = useCallback((action: UndoAction) => {