feat: automatically sign out after 403 response

frontend/lib/main.dart

@@ -7,6 +7,8 @@ import 'services/notification_service.dart';
 import 'services/auth_service.dart';
 import 'services/user_service.dart';
 
+final GlobalKey<NavigatorState> navigatorKey = GlobalKey<NavigatorState>();
+
 void main() async {
   WidgetsFlutterBinding.ensureInitialized();
   await Firebase.initializeApp(options: DefaultFirebaseOptions.currentPlatform);
@@ -21,6 +23,7 @@ class MyApp extends StatelessWidget {
       title: 'Wesal',
       theme: ThemeData(primarySwatch: Colors.blue, fontFamily: 'Roboto'),
       home: SplashScreen(),
+      navigatorKey: navigatorKey,
       debugShowCheckedModeBanner: false,
     );
   }

frontend/lib/services/auth_service.dart

@@ -1,7 +1,9 @@
 import 'dart:convert';
+import 'package:flutter/material.dart';
 import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import 'package:http/http.dart' as http;
 import '../constants/api_constants.dart';
+import '../main.dart';
 
 class AuthService {
   static const FlutterSecureStorage _storage = FlutterSecureStorage();
@@ -87,4 +89,13 @@ class AuthService {
   static Future<void> clearUserData() async {
     await _storage.delete(key: _userDataKey);
   }
+
+  static Future<void> handleAuthenticationError() async {
+    await logout();
+    
+    final context = navigatorKey.currentContext;
+    if (context != null && context.mounted) {
+      Navigator.of(context).pushNamedAndRemoveUntil('/', (route) => false);
+    }
+  }
 }

frontend/lib/services/invitations_service.dart

@@ -2,6 +2,7 @@ import 'dart:convert';
 import '../constants/api_constants.dart';
 import '../models/invitation_models.dart';
 import 'http_service.dart';
+import 'auth_service.dart';
 
 class InvitationsService {
   static Future<Map<String, dynamic>> getAllInvitations() async {
@@ -23,16 +24,12 @@ class InvitationsService {
                 invitationsResponse.message ?? 'Failed to fetch invitations',
           };
         }
-      } else if (response.statusCode == 401) {
+      } else if (response.statusCode == 401 || response.statusCode == 403) {
+        await AuthService.handleAuthenticationError();
         return {
           'success': false,
           'message': 'Session expired. Please login again.',
         };
-      } else if (response.statusCode == 403) {
-        return {
-          'success': false,
-          'message': 'Access denied. Invalid credentials.',
-        };
       } else {
         return {
           'success': false,

frontend/lib/services/post_service.dart

@@ -1,15 +1,15 @@
 import 'dart:convert';
 import '../models/post_models.dart';
 import 'http_service.dart';
+import 'auth_service.dart';
 
 class PostService {
   static Future<Map<String, dynamic>> getAllPosts() async {
     try {
       final response = await HttpService.get('/posts/all');
 
-      final responseData = jsonDecode(response.body);
-
       if (response.statusCode == 200) {
+        final responseData = jsonDecode(response.body);
         return {
           'success': responseData['status'] ?? false,
           'message': responseData['message'] ?? '',
@@ -19,12 +19,28 @@ class PostService {
                   .toList() ??
               [],
         };
-      } else {
+      } else if (response.statusCode == 401 || response.statusCode == 403) {
+        await AuthService.handleAuthenticationError();
         return {
           'success': false,
-          'message': responseData['message'] ?? 'Failed to fetch posts',
+          'message': 'Session expired. Please login again.',
           'posts': <Post>[],
         };
+      } else {
+        try {
+          final responseData = jsonDecode(response.body);
+          return {
+            'success': false,
+            'message': responseData['message'] ?? 'Failed to fetch posts',
+            'posts': <Post>[],
+          };
+        } catch (e) {
+          return {
+            'success': false,
+            'message': 'Server error (${response.statusCode})',
+            'posts': <Post>[],
+          };
+        }
       }
     } catch (e) {
       print('Error fetching posts: $e');

frontend/lib/services/user_service.dart

@@ -21,16 +21,12 @@ class UserService {
         final data = jsonDecode(response.body);
         await AuthService.saveUserData(data);
         return {'success': true, 'data': data};
-      } else if (response.statusCode == 401) {
+      } else if (response.statusCode == 401 || response.statusCode == 403) {
+        await AuthService.handleAuthenticationError();
         return {
           'success': false,
           'message': 'Session expired. Please login again.',
         };
-      } else if (response.statusCode == 403) {
-        return {
-          'success': false,
-          'message': 'Access denied. Invalid credentials.',
-        };
       } else {
         return {
           'success': false,