fix(frontend): align types + UI to backend contract (docs/api.md @ dd5c508)

Backend pushed the authoritative contract in docs/api.md and tightened
the error envelope via a global HTTPException handler (dd5c508). This
commit folds the frontend onto that contract — every drift flagged by
the code-reviewer MAJOR is closed.

Types (src/types/api.ts)
- User: `id` → `user_id`; `display_name` is `string | null`; add
  `permissions: string[]` and `groups: string[]`; drop `engagement_id`
  and `engagement_name` (not part of CurrentUser).
- Engagement: drop `name`, `client_name` is non-null `string`; status
  enum aligned to `draft | active | closed | archived`; `c2_type` is
  non-null `C2Type`; drop `created_at` (not in EngagementRead v1).
- EngagementCreate body: `client_name` required, plus optional
  `description`, `c2_type`, `start_date`, `end_date`. No `name`.
- Replace ApiError + ApiValidationError with a single uniform envelope:
  `{ error: string, message: string, details?: PydanticErrorItem[] }`,
  matching the new HTTPException handler. PydanticErrorItem is the
  per-field shape on 422 (`{ loc, msg, type }`).

Fetch client (src/lib/api.ts)
- `bodyAsApiError` now recognizes the uniform envelope by shape
  (error+message strings). Anything else returns null so callers fall
  back to a generic message — keeps us robust if the backend ever
  emits a non-JSON response.

Engagements API (src/screens/engagements/engagementsApi.ts)
- Drop the `{ items: [] }` envelope tolerance — backend serves a bare
  `Engagement[]`.
- Hit `/engagements/` with trailing slash explicitly; backend now sets
  `strict_slashes=False` but staying consistent with docs/api.md.

EngagementsPage
- Status tone map switched to the new enum (`draft → pending`,
  `closed → soc`).
- Drop "Name" column. `client_name` is the primary identifier; the
  description column replaces the now-meaningless name field.
- `c2_type` is non-null, so no nullable rendering path.

EngagementCreateDialog
- Drop `name` field. New required field is `client_name`; add a
  `c2_type` select (default `mythic`); brief textarea stays optional.
- `mapValidationErrors` now reads `body.details[*].loc` (last segment
  matches the form field) — direct alignment with the backend's new
  shape after dd5c508.
- 401 still surfaces "Session expirée"; 403 gains a dedicated message;
  other errors fall back to a capitalized backend `message` when
  available, then to a generic French string.

Sidebar
- Display fallback: `user.display_name ?? user.username` (now nullable).
- Drop the `ENG · {engagement_name}` line; show `user.username` (the
  email) as the secondary identity instead.

LoginPage
- Field label "Username" → "Email or username" so RT users with email
  accounts find the field semantically obvious (per docs/api.md note
  on the username/email mapping).

Tests (Vitest, 14 cases, all green)
- Refreshed fixtures to the new shapes (no more `name`, no
  `created_at`, status `draft`, envelopes carry `error`+`message`).
- New 422 test exercises the `details[*].loc` mapping shape.
- New 401 test on the dialog covers the top-of-form alert path.

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

@@ -12,7 +12,6 @@ describe('EngagementsPage', () => {
 
   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 />);
@@ -26,7 +25,9 @@ describe('EngagementsPage', () => {
   });
 
   it('renders the error state on 500', async () => {
-    fetchMock = installFetchMock([{ status: 500, body: { detail: 'internal_error' } }]);
+    fetchMock = installFetchMock([
+      { status: 500, body: { error: 'internal_error', message: 'boom' } },
+    ]);
     renderWithProviders(<EngagementsPage />);
     await waitFor(() => {
       expect(screen.getByText(/fetch failed/i)).toBeInTheDocument();
@@ -41,21 +42,19 @@ describe('EngagementsPage', () => {
         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();
+    await screen.findByText('Acme');
     expect(screen.getByText('MYTHIC')).toBeInTheDocument();
+    expect(screen.getByText(/active/)).toBeInTheDocument();
   });
 });