test(setup): stub websocket addListener/removeListener in the global mock

BackgroundTasksWidget (mounted globally in App) subscribes via addListener/removeListener from api/websocket, but the global test mock didn't export them, so every test that renders <App/> threw on mount. Add the two stubs. (Surfaced now that the page-pattern check passes and the client test step actually runs.)

client/tests/setup.ts

@@ -11,6 +11,8 @@ vi.mock('../src/api/websocket', () => ({
   getSocketId: vi.fn(() => null),
   setRefetchCallback: vi.fn(),
   setPreReconnectHook: vi.fn(),
+  addListener: vi.fn(),
+  removeListener: vi.fn(),
 }));
 
 // MSW lifecycle