feat(front): implémenter la page create-password du ticket #118

Ajoute le flux frontend de création initiale du mot de passe via token email (route /create-password, vérification de token et soumission /auth/create-password), avec redirection login en succès et gestion neutre des liens invalides/expirés.

Made-with: Cursor

frontend/lib/config/app_router.dart

@@ -18,6 +18,7 @@ import '../screens/auth/am_register_step1_screen.dart';
 import '../screens/auth/am_register_step2_screen.dart';
 import '../screens/auth/am_register_step3_screen.dart';
 import '../screens/auth/am_register_step4_screen.dart';
+import '../screens/auth/create_password_screen.dart';
 import '../screens/home/home_screen.dart';
 import '../screens/administrateurs/admin_dashboardScreen.dart';
 import '../screens/gestionnaire/gestionnaire_dashboard_screen.dart';
@@ -49,6 +50,11 @@ class AppRouter {
         path: '/register-choice',
         builder: (BuildContext context, GoRouterState state) => const RegisterChoiceScreen(),
       ),
+      GoRoute(
+        path: '/create-password',
+        builder: (BuildContext context, GoRouterState state) =>
+            CreatePasswordScreen(token: state.uri.queryParameters['token']),
+      ),
       GoRoute(
         path: '/home',
         builder: (BuildContext context, GoRouterState state) => const HomeScreen(),