fix(frontend): i18n reactive badges + fr-FR dates on detail/admin + FR footer (design-review)

- StatusBadge/SimulationStatusBadge: swap bare i18n.t() helper for
  useTranslation() hook so badges re-render reactively on i18n init
- EngagementDetailPage: formatDate() on start_date/end_date/created_at
  (was: raw ISO string, formatDateTime with noisy 00:00:00)
- UsersAdminPage: formatDate() on created_at column (was: raw ISO string)
- Layout footer: t('nav.footer') with FR string in fr.json

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

frontend/src/components/StatusBadge.tsx

@@ -1,5 +1,5 @@
+import { useTranslation } from 'react-i18next';
 import type { EngagementStatus } from '@/api/types';
-import { engagementStatusLabel } from '@/i18n/status';
 
 const STYLES: Record<EngagementStatus, string> = {
   planned: 'bg-warn-soft text-warn border border-warn',
@@ -8,13 +8,14 @@ const STYLES: Record<EngagementStatus, string> = {
 };
 
 export function StatusBadge({ status }: { status: EngagementStatus }): JSX.Element {
+  const { t } = useTranslation();
   return (
     <span
       className={`inline-flex items-center rounded-pill px-3 py-[6px] text-[14px] leading-[1.3] font-medium ${STYLES[status]}`}
       data-testid="status-badge"
       data-status={status}
     >
-      {engagementStatusLabel(status)}
+      {t(`status.engagement.${status}`)}
     </span>
   );
 }