fix: mock system api in layout tests to prevent network crashes

- Mocked `getNotifications` and `checkUpdates` in `Layout.test.tsx`
- Prevents `UND_ERR_INVALID_ARG` errors caused by unmocked `undici` network requests in JSDOM
- Ensures clean test execution for `Layout` and child components

frontend/src/components/__tests__/Layout.test.tsx

@@ -32,6 +32,14 @@ vi.mock('../../api/featureFlags', () => ({
   }),
 }))
 
+// Mock System API to prevent unhandled network requests
+vi.mock('../../api/system', () => ({
+  getNotifications: vi.fn().mockResolvedValue([]),
+  markNotificationRead: vi.fn(),
+  markAllNotificationsRead: vi.fn(),
+  checkUpdates: vi.fn().mockResolvedValue({ available: false }),
+}))
+
 const renderWithProviders = (children: ReactNode) => {
   const queryClient = new QueryClient({
     defaultOptions: {