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/EngagementCreateDialog.test.tsx

@@ -0,0 +1,77 @@
+import { describe, it, expect, afterEach, vi } from 'vitest';
+import { screen, fireEvent, waitFor } from '@testing-library/react';
+import { EngagementCreateDialog } from './EngagementCreateDialog';
+import { installFetchMock, renderWithProviders } from '@/test/testUtils';
+
+describe('EngagementCreateDialog', () => {
+  let fetchMock: ReturnType<typeof installFetchMock>;
+
+  afterEach(() => {
+    fetchMock?.restore();
+  });
+
+  it('rejects empty name client-side without calling the backend', () => {
+    fetchMock = installFetchMock([]);
+    renderWithProviders(<EngagementCreateDialog onClose={vi.fn()} />);
+    fireEvent.click(screen.getByRole('button', { name: /arm engagement/i }));
+    expect(screen.getByText(/nom requis/i)).toBeInTheDocument();
+    expect(fetchMock.calls).toHaveLength(0);
+  });
+
+  it('maps 422 Pydantic errors to per-field messages', async () => {
+    fetchMock = installFetchMock([
+      {
+        status: 422,
+        body: {
+          detail: [
+            {
+              loc: ['body', 'name'],
+              msg: 'String should have at least 3 characters',
+              type: 'string_too_short',
+            },
+          ],
+        },
+      },
+    ]);
+    renderWithProviders(<EngagementCreateDialog onClose={vi.fn()} />);
+
+    fireEvent.change(screen.getByLabelText(/engagement name/i), { target: { value: 'AB' } });
+    fireEvent.click(screen.getByRole('button', { name: /arm engagement/i }));
+
+    await waitFor(() => {
+      expect(screen.getByText(/at least 3 characters/i)).toBeInTheDocument();
+    });
+  });
+
+  it('invalidates the engagements query and closes on success', async () => {
+    const onClose = vi.fn();
+    fetchMock = installFetchMock([
+      {
+        status: 201,
+        body: {
+          id: 'eng_new',
+          name: 'OPERATION ZETA',
+          client_name: null,
+          description: null,
+          status: 'planning',
+          c2_type: null,
+          start_date: null,
+          end_date: null,
+          created_at: '2026-05-23T08:00:00Z',
+        },
+      },
+    ]);
+    renderWithProviders(<EngagementCreateDialog onClose={onClose} />);
+
+    fireEvent.change(screen.getByLabelText(/engagement name/i), {
+      target: { value: 'OPERATION ZETA' },
+    });
+    fireEvent.click(screen.getByRole('button', { name: /arm engagement/i }));
+
+    await waitFor(() => {
+      expect(onClose).toHaveBeenCalledTimes(1);
+    });
+    expect(fetchMock.calls[0]?.url).toBe('/api/v1/engagements');
+    expect(fetchMock.calls[0]?.init?.method).toBe('POST');
+  });
+});