feat(frontend): c2 config card + execute modal (sprint 8 phase 1)

- frontend/src/api/c2.ts: 6 typed API calls (getC2Config, putC2Config,
  deleteC2Config, testC2Config, listCallbacks, executeC2) following the
  frozen M1+M2 backend contracts
- frontend/src/api/types.ts: C2Config, C2ConfigInput, C2TestResult,
  C2Callback, C2CallbacksResponse, C2Task, C2ExecuteInput, C2ExecuteResponse
- frontend/src/hooks/useC2.ts: useC2Config, useUpdateC2Config,
  useDeleteC2Config, useTestC2Config, useC2Callbacks, useExecuteC2
- frontend/src/components/C2ConfigCard.tsx: engagement-scoped C2 config
  card (url + write-only token + verify-tls + save/delete/test-connection),
  503 disabled state, ConfirmDialog on delete
- frontend/src/components/ExecuteViaC2Modal.tsx: callback picker table
  (mono data cells), commands textarea pre-filled from rt.commands,
  Launch disabled until row selected + non-empty commands
- frontend/src/pages/EngagementFormPage.tsx: embed C2ConfigCard in edit
  mode only, admin+redteam only (canEditEngagements gate)
- frontend/src/pages/SimulationFormPage.tsx: Execute via C2 button in RT
  card, visible only when !isDone && canEditRT && hasC2Config; opens modal
- Tests: 33 new tests across api/c2, components/C2ConfigCard,
  components/ExecuteViaC2Modal, EngagementFormPage, SimulationFormPage
  (172 total, 139 baseline + 33 new, all passing)

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

frontend/tests/SimulationFormPage.test.tsx

@@ -67,6 +67,7 @@ describe('SimulationFormPage — redteam mode (edit existing)', () => {
     mockRole = 'redteam';
     mock = new MockAdapter(apiClient);
     mock.onGet('/simulations/7').reply(200, BASE_SIM);
+    mock.onGet('/engagements/42/c2-config').reply(404);
   });
 
   afterEach(() => {
@@ -154,6 +155,8 @@ describe('SimulationFormPage — SOC role + pending (blocked)', () => {
     mockRole = 'soc';
     mock = new MockAdapter(apiClient);
     mock.onGet('/simulations/7').reply(200, BASE_SIM);
+    // SOC role: useC2Config disabled (canEditRT=false), so no request expected — stub anyway
+    mock.onGet('/engagements/42/c2-config').reply(404);
   });
 
   afterEach(() => {
@@ -202,6 +205,7 @@ describe('SimulationFormPage — SOC role + review_required (can edit SOC fields
     mockRole = 'soc';
     mock = new MockAdapter(apiClient);
     mock.onGet('/simulations/7').reply(200, { ...BASE_SIM, status: 'review_required' });
+    mock.onGet('/engagements/42/c2-config').reply(404);
   });
 
   afterEach(() => {
@@ -273,3 +277,58 @@ describe('SimulationFormPage — new simulation', () => {
     expect(screen.getByRole('button', { name: /Create simulation/i })).toBeInTheDocument();
   });
 });
+
+describe('SimulationFormPage — Execute via C2 button visibility', () => {
+  let mock: MockAdapter;
+
+  beforeEach(() => {
+    mockRole = 'redteam';
+    mock = new MockAdapter(apiClient);
+    mock.onGet('/simulations/7').reply(200, BASE_SIM);
+  });
+
+  afterEach(() => {
+    mock.restore();
+  });
+
+  it('shows Execute via C2 button when c2 config exists', async () => {
+    mock.onGet('/engagements/42/c2-config').reply(200, {
+      has_token: true,
+      url: 'https://mythic.lab:7443',
+      verify_tls: true,
+    });
+    renderWithProviders(<EditPage />, {
+      routerProps: { initialEntries: ['/engagements/42/simulations/7/edit'] },
+    });
+    await waitFor(() => {
+      expect(screen.getByTestId('c2-execute-btn')).toBeInTheDocument();
+    });
+  });
+
+  it('hides Execute via C2 button when no c2 config (404)', async () => {
+    mock.onGet('/engagements/42/c2-config').reply(404);
+    renderWithProviders(<EditPage />, {
+      routerProps: { initialEntries: ['/engagements/42/simulations/7/edit'] },
+    });
+    await waitFor(() => {
+      expect(screen.getByLabelText(/^Name/i)).not.toBeDisabled();
+    });
+    expect(screen.queryByTestId('c2-execute-btn')).toBeNull();
+  });
+
+  it('hides Execute via C2 button when simulation is done', async () => {
+    mock.onGet('/simulations/7').reply(200, { ...BASE_SIM, status: 'done' });
+    mock.onGet('/engagements/42/c2-config').reply(200, {
+      has_token: true,
+      url: 'https://mythic.lab:7443',
+      verify_tls: true,
+    });
+    renderWithProviders(<EditPage />, {
+      routerProps: { initialEntries: ['/engagements/42/simulations/7/edit'] },
+    });
+    await waitFor(() => {
+      expect(screen.getByTestId('reopen-btn')).toBeInTheDocument();
+    });
+    expect(screen.queryByTestId('c2-execute-btn')).toBeNull();
+  });
+});