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/providers/auth_provider.dart

@@ -0,0 +1,52 @@
+import 'package:flutter/material.dart';
+import '../services/auth_service.dart';
+
+class AuthProvider extends ChangeNotifier {
+  bool _isAuthenticated = false;
+  String? _role;
+  String? _errorMessage;
+  bool _isLoading = false;
+
+  bool get isAuthenticated => _isAuthenticated;
+  String? get role => _role;
+  String? get errorMessage => _errorMessage;
+  bool get isLoading => _isLoading;
+
+  Future<bool> login(String username, String password) async {
+    _isLoading = true;
+    _errorMessage = null;
+    notifyListeners();
+
+    try {
+      final data = await AuthService.login(username, password);
+      final token = data['token'] as String;
+      final roles = data['roles'] as List<dynamic>;
+      final role = roles.isNotEmpty ? roles.first as String : 'ROLE_USER';
+
+      await AuthService.saveToken(token, role);
+      _isAuthenticated = true;
+      _role = role;
+      return true;
+    } catch (e) {
+      _errorMessage = 'Identifiants incorrects. Vérifiez votre nom d\'utilisateur et mot de passe.';
+      return false;
+    } finally {
+      _isLoading = false;
+      notifyListeners();
+    }
+  }
+
+  Future<void> logout() async {
+    await AuthService.logout();
+    _isAuthenticated = false;
+    _role = null;
+    notifyListeners();
+  }
+
+  Future<void> checkAuth() async {
+    final token = await AuthService.getToken();
+    _isAuthenticated = token != null;
+    _role = await AuthService.getRole();
+    notifyListeners();
+  }
+}