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/i18n/fr.json

@@ -23,7 +23,8 @@
     "templates": "Templates",
     "users": "Utilisateurs",
     "signOut": "Se déconnecter",
-    "brand": "Mimic"
+    "brand": "Mimic",
+    "footer": "Mimic — Outillage interne Purple Team. Engagements autorisés uniquement."
   },
   "auth": {
     "login": {