test(frontend): Vitest coverage on sprint 1 wiring

Adds a small test harness (src/test/testUtils.tsx):
- renderWithProviders mounts a fresh QueryClient (no retries, no cache)
  + MemoryRouter so screens using useNavigate / <Link> don't crash.
- installFetchMock(responses[]) replaces globalThis.fetch with a typed
  sequence of canned responses and records call URLs + init.

Specs (10 cases, all green):

LoginPage.test.tsx
- happy path: submit posts to /api/v1/auth/login with credentials:'include',
  correct JSON body shape (username/password).
- 401 surfaces "Identifiants invalides" and does NOT leak the backend
  detail string.
- empty submit is intercepted by HTML5 `required` — no fetch fires.

EngagementsPage.test.tsx
- loading row renders while /engagements is in flight.
- empty state renders on 200 [].
- error state + Retry button render on 500.
- populated table renders the snake_case fields correctly (name,
  client_name, c2_type uppercased).

EngagementCreateDialog.test.tsx
- client-side validation: empty name blocks submission, no fetch fires.
- 422 Pydantic error on the `name` field maps to the inline message
  next to the input.
- 201 success triggers onClose() and POSTs to /api/v1/engagements.

frontend/src/screens/engagements/EngagementsPage.test.tsx

@@ -0,0 +1,61 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { screen, waitFor } from '@testing-library/react';
+import { EngagementsPage } from './EngagementsPage';
+import { installFetchMock, renderWithProviders } from '@/test/testUtils';
+
+describe('EngagementsPage', () => {
+  let fetchMock: ReturnType<typeof installFetchMock>;
+
+  afterEach(() => {
+    fetchMock?.restore();
+  });
+
+  it('shows the loading row while the engagements query is pending', () => {
+    fetchMock = installFetchMock([]); // never resolve in this test
+    // Replace with a long-lived promise so the query sits in pending state.
+    const pending: typeof fetch = () => new Promise(() => null);
+    globalThis.fetch = pending;
+    renderWithProviders(<EngagementsPage />);
+    expect(screen.getByText(/fetching engagements/i)).toBeInTheDocument();
+  });
+
+  it('renders the empty state when the list is empty', async () => {
+    fetchMock = installFetchMock([{ status: 200, body: [] }]);
+    renderWithProviders(<EngagementsPage />);
+    await screen.findByText(/no engagements yet/i);
+  });
+
+  it('renders the error state on 500', async () => {
+    fetchMock = installFetchMock([{ status: 500, body: { detail: 'internal_error' } }]);
+    renderWithProviders(<EngagementsPage />);
+    await waitFor(() => {
+      expect(screen.getByText(/fetch failed/i)).toBeInTheDocument();
+    });
+    expect(screen.getByRole('button', { name: /retry/i })).toBeInTheDocument();
+  });
+
+  it('renders rows when the backend returns engagements', async () => {
+    fetchMock = installFetchMock([
+      {
+        status: 200,
+        body: [
+          {
+            id: 'eng_1',
+            name: 'OPERATION ALPHA',
+            client_name: 'Acme',
+            description: null,
+            status: 'active',
+            c2_type: 'mythic',
+            start_date: '2026-05-20',
+            end_date: '2026-05-30',
+            created_at: '2026-05-20T10:00:00Z',
+          },
+        ],
+      },
+    ]);
+    renderWithProviders(<EngagementsPage />);
+    await screen.findByText('OPERATION ALPHA');
+    expect(screen.getByText('Acme')).toBeInTheDocument();
+    expect(screen.getByText('MYTHIC')).toBeInTheDocument();
+  });
+});