ISSUE 5: add undo functionality

src/components/DockBuilder/DockBuilder.jsx

@@ -497,7 +497,12 @@ export const DockBuilder = () => {
   }, [editor, selectedItems]);
 
   const onUndoClick = useCallback(() => {
-    // TODO: Undo
+    const undoResult = stack.undo();
+    if (undoResult && undoResult.currentState) {
+      editorMemo.loadFromJSON(undoResult.currentState, () => {
+        editorMemo.renderAll();
+      });
+    }
   }, [editorMemo]);
 
   const onRedoClick = useCallback(() => {