feat(frontend): init Flutter project with login screen and JWT auth

- Projet Flutter créé manuellement (pubspec.yaml, structure lib/)
- Écran de connexion complet avec validation, gestion erreurs, spinner
- ApiClient Dio avec intercepteur JWT automatique
- AuthProvider (Provider) + AuthService (shared_preferences)
- Routing GoRouter avec redirection auth/non-auth
- DashboardScreen placeholder
- Rapport PFE : ajout Chapitre 5 Frontend (architecture, JWT, login)

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

frontend/lib/services/api_client.dart

@@ -0,0 +1,33 @@
+import 'package:dio/dio.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+class ApiClient {
+  static const String baseUrl = 'http://192.168.100.33:8090/api';
+
+  static final Dio _dio = Dio(BaseOptions(
+    baseUrl: baseUrl,
+    connectTimeout: const Duration(seconds: 10),
+    receiveTimeout: const Duration(seconds: 15),
+    headers: {'Content-Type': 'application/json'},
+  ))
+    ..interceptors.add(_AuthInterceptor());
+
+  static Dio get instance => _dio;
+}
+
+class _AuthInterceptor extends Interceptor {
+  @override
+  void onRequest(RequestOptions options, RequestInterceptorHandler handler) async {
+    final prefs = await SharedPreferences.getInstance();
+    final token = prefs.getString('jwt_token');
+    if (token != null) {
+      options.headers['Authorization'] = 'Bearer $token';
+    }
+    handler.next(options);
+  }
+
+  @override
+  void onError(DioException err, ErrorInterceptorHandler handler) {
+    handler.next(err);
+  }
+}

frontend/lib/services/auth_service.dart

@@ -0,0 +1,35 @@
+import 'package:dio/dio.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'api_client.dart';
+
+class AuthService {
+  static Future<Map<String, dynamic>> login(String username, String password) async {
+    final response = await ApiClient.instance.post('/auth/signin', data: {
+      'username': username,
+      'password': password,
+    });
+    return response.data as Map<String, dynamic>;
+  }
+
+  static Future<void> saveToken(String token, String role) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString('jwt_token', token);
+    await prefs.setString('user_role', role);
+  }
+
+  static Future<String?> getToken() async {
+    final prefs = await SharedPreferences.getInstance();
+    return prefs.getString('jwt_token');
+  }
+
+  static Future<String?> getRole() async {
+    final prefs = await SharedPreferences.getInstance();
+    return prefs.getString('user_role');
+  }
+
+  static Future<void> logout() async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.remove('jwt_token');
+    await prefs.remove('user_role');
+  }
+}