feat(frontend): i18n common + nav + auth (Layout, LoginPage, ProtectedRoute)

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

frontend/src/components/ProtectedRoute.tsx

@@ -1,5 +1,6 @@
 import { useEffect } from 'react';
 import { Navigate, Outlet, useLocation } from 'react-router-dom';
+import { useTranslation } from 'react-i18next';
 import { useAuth } from '@/hooks/useAuth';
 import { useToast } from '@/hooks/useToast';
 import type { Role } from '@/api/types';
@@ -23,6 +24,7 @@ export function ProtectedRoute({
 }: ProtectedRouteProps): JSX.Element {
   const { user, status } = useAuth();
   const { push } = useToast();
+  const { t } = useTranslation();
   const location = useLocation();
 
   const roleDenied = Boolean(
@@ -31,12 +33,12 @@ export function ProtectedRoute({
 
   useEffect(() => {
     if (roleDenied) {
-      push('Accès refusé', 'error');
+      push(t('auth.forbidden'), 'error');
     }
-  }, [roleDenied, push]);
+  }, [roleDenied, push, t]);
 
   if (status === 'loading') {
-    return <LoadingState label="Loading session…" />;
+    return <LoadingState label={t('auth.loadingSession')} />;
   }
 
   if (status === 'unauthenticated' || !user) {