feat: ExportEngagementButton + exports API client

Add split-button dropdown [Export ▼] on EngagementDetailPage that
downloads engagement as Markdown, CSV, or PDF via
GET /api/engagements/<id>/export?format=md|csv|pdf.

Both halves open the dropdown (no default left-click action).
RBAC-gated with canEditEngagements (admin + redteam only).
Loading state per item, toast on error, click-outside + Escape close.

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

frontend/src/pages/EngagementDetailPage.tsx

@@ -6,6 +6,7 @@ import { LoadingState } from '@/components/LoadingState';
 import { ErrorState } from '@/components/ErrorState';
 import { StatusBadge } from '@/components/StatusBadge';
 import { SimulationList } from '@/components/SimulationList';
+import { ExportEngagementButton } from '@/components/ExportEngagementButton';
 
 export function EngagementDetailPage(): JSX.Element {
   const { id } = useParams<{ id: string }>();
@@ -43,9 +44,12 @@ export function EngagementDetailPage(): JSX.Element {
           </div>
         </div>
         {canEditEngagements ? (
-          <Link to={`/engagements/${eng.id}/edit`} className="btn-outline">
-            Edit
-          </Link>
+          <div className="flex items-center gap-sm">
+            <ExportEngagementButton engagementId={eng.id} />
+            <Link to={`/engagements/${eng.id}/edit`} className="btn-outline">
+              Edit
+            </Link>
+          </div>
         ) : null}
       </header>