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/pages/EngagementDetailPage.tsx

@@ -12,7 +12,7 @@ import { SimulationList } from '@/components/SimulationList';
 import { ExportEngagementButton } from '@/components/ExportEngagementButton';
 import { BackLink } from '@/components/BackLink';
 import { Tabs } from '@/components/Tabs';
-import { formatDateTime } from '@/lib/format';
+import { formatDate } from '@/lib/format';
 
 type TabId = 'description' | 'simulations';
 
@@ -78,13 +78,13 @@ export function EngagementDetailPage(): JSX.Element {
             <header className="flex items-start justify-between gap-md">
               <dl className="grid grid-cols-2 gap-x-lg gap-y-xxs text-caption-md">
                 <dt className="text-graphite">{t('engagement.detail.schedule.startDate')}</dt>
-                <dd className="font-mono">{eng.start_date}</dd>
+                <dd className="font-mono">{formatDate(eng.start_date)}</dd>
                 <dt className="text-graphite">{t('engagement.detail.schedule.endDate')}</dt>
-                <dd className="font-mono">{eng.end_date ?? '—'}</dd>
+                <dd className="font-mono">{eng.end_date ? formatDate(eng.end_date) : '—'}</dd>
                 <dt className="text-graphite">{t('engagement.detail.schedule.status')}</dt>
                 <dd><StatusBadge status={eng.status} /></dd>
                 <dt className="text-graphite">{t('engagement.detail.schedule.createdAt')}</dt>
-                <dd className="font-mono">{formatDateTime(eng.created_at)}</dd>
+                <dd className="font-mono">{formatDate(eng.created_at)}</dd>
               </dl>
               {canEditEngagements ? (
                 <Link to={`/engagements/${eng.id}/edit`} className="btn-outline shrink-0">