fix(test): add getState to useCanvasStore mock in ContextMenu keyboard test

PR #1781 introduced useCanvasStore.getState() call in ContextMenu.tsx
(line 169) but the existing Vitest mock for useCanvasStore in the keyboard
test file lacked a getState method, causing:
  TypeError: useCanvasStore.getState is not a function

Fix: attach getState: () => mockStore to the mock using Object.assign
so the static method is available alongside the selector fn.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

canvas/src/components/__tests__/ContextMenu.keyboard.test.tsx

@@ -49,8 +49,9 @@ const mockStore = {
 };
 
 vi.mock("@/store/canvas", () => ({
-  useCanvasStore: vi.fn(
-    (selector: (s: typeof mockStore) => unknown) => selector(mockStore)
+  useCanvasStore: Object.assign(
+    (selector: (s: typeof mockStore) => unknown) => selector(mockStore),
+    { getState: () => mockStore }
   ),
 }));