feat: add guest mode — browse events without login

New file: lib/core/auth/auth_guard.dart
  Static AuthGuard class with isGuest flag and requireLogin() helper
  that shows a login prompt bottom sheet when guests try protected actions.

login_screen.dart / desktop_login_screen.dart:
  Added "Continue as Guest" button below sign-up link.
  Sets AuthGuard.isGuest = true, then navigates to HomeScreen.

api_client.dart:
  _buildAuthBody() and GET auth check no longer throw when token is missing.
  If no token (guest), request proceeds without auth — backend decides.

home_screen.dart:
  Bottom nav guards: tapping Contribute (index 2) or Profile (index 3)
  as guest shows login prompt instead of navigating.

auth_service.dart:
  AuthGuard.setGuest(false) called on successful login AND register
  so guest flag is always cleared when user authenticates.

Guest CAN: browse home, calendar, search, filter, view event details.
Guest CANNOT: contribute, view profile, book events (prompts login).

lib/core/api/api_client.dart

@@ -81,11 +81,11 @@ class ApiClient {
     if (requiresAuth) {
       final token = await TokenStorage.getToken();
       final username = await TokenStorage.getUsername();
-      if (token == null || username == null) {
-        throw Exception('Authentication required');
+      if (token != null && username != null) {
+        finalParams['token'] = token;
+        finalParams['username'] = username;
       }
-      finalParams['token'] = token;
-      finalParams['username'] = username;
+      // Guest mode: proceed without token — let backend decide
     }
 
     if (params != null) finalParams.addAll(params);
@@ -103,7 +103,7 @@ class ApiClient {
     return _handleResponse(url, response, finalParams);
   }
 
-  /// Build request body and attach token + username if required
+  /// Build request body and attach token + username if available
   Future<Map<String, dynamic>> _buildAuthBody(Map<String, dynamic>? body, bool requiresAuth) async {
     final Map<String, dynamic> finalBody = {};
 
@@ -111,12 +111,11 @@ class ApiClient {
       final token = await TokenStorage.getToken();
       final username = await TokenStorage.getUsername();
 
-      if (token == null || username == null) {
-        throw Exception('Authentication required');
+      if (token != null && username != null) {
+        finalBody['token'] = token;
+        finalBody['username'] = username;
       }
-
-      finalBody['token'] = token;
-      finalBody['username'] = username;
+      // Guest mode: proceed without token — let backend decide
     }
 
     if (body != null) finalBody.addAll(body);

lib/core/auth/auth_guard.dart

@@ -0,0 +1,71 @@
+// lib/core/auth/auth_guard.dart
+import 'package:flutter/material.dart';
+import '../../screens/login_screen.dart';
+
+class AuthGuard {
+  static bool _isGuest = false;
+
+  static bool get isGuest => _isGuest;
+  static bool get isLoggedIn => !_isGuest;
+
+  static void setGuest(bool value) => _isGuest = value;
+
+  /// Call before any action that requires login.
+  /// Returns true if logged in (proceed). Returns false if guest (shows prompt).
+  static bool requireLogin(BuildContext context,
+      {String reason = 'This feature requires an account.'}) {
+    if (!_isGuest) return true;
+    _showLoginPrompt(context, reason);
+    return false;
+  }
+
+  static void _showLoginPrompt(BuildContext context, String reason) {
+    showModalBottomSheet(
+      context: context,
+      shape: const RoundedRectangleBorder(
+        borderRadius: BorderRadius.vertical(top: Radius.circular(20)),
+      ),
+      builder: (ctx) => Padding(
+        padding: const EdgeInsets.all(24),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            const Icon(Icons.lock_outline, size: 48, color: Color(0xFF0B63D6)),
+            const SizedBox(height: 16),
+            Text(reason,
+                textAlign: TextAlign.center,
+                style: const TextStyle(
+                    fontSize: 16, fontWeight: FontWeight.w600)),
+            const SizedBox(height: 8),
+            const Text('Sign in or create an account to continue.',
+                textAlign: TextAlign.center,
+                style: TextStyle(color: Colors.grey, fontSize: 14)),
+            const SizedBox(height: 24),
+            SizedBox(
+              width: double.infinity,
+              height: 48,
+              child: ElevatedButton(
+                style: ElevatedButton.styleFrom(
+                  backgroundColor: const Color(0xFF0B63D6),
+                  shape: RoundedRectangleBorder(
+                      borderRadius: BorderRadius.circular(12)),
+                ),
+                onPressed: () {
+                  Navigator.of(ctx).pop();
+                  Navigator.of(context).pushAndRemoveUntil(
+                    MaterialPageRoute(builder: (_) => const LoginScreen()),
+                    (route) => false,
+                  );
+                },
+                child: const Text('Sign In',
+                    style: TextStyle(
+                        color: Colors.white, fontWeight: FontWeight.w700)),
+              ),
+            ),
+            const SizedBox(height: 12),
+          ],
+        ),
+      ),
+    );
+  }
+}

lib/features/auth/services/auth_service.dart

@@ -3,6 +3,7 @@ import 'package:flutter/foundation.dart' show kDebugMode, debugPrint;
 import 'package:shared_preferences/shared_preferences.dart';
 import '../../../core/api/api_client.dart';
 import '../../../core/api/api_endpoints.dart';
+import '../../../core/auth/auth_guard.dart';
 import '../../../core/storage/token_storage.dart';
 import '../models/user_model.dart';
 
@@ -33,6 +34,9 @@ class AuthService {
       // candidate display name (server username or email fallback)
       final displayCandidate = serverUsername ?? savedEmail;
 
+      // clear guest mode on successful login
+      AuthGuard.setGuest(false);
+
       // save token (TokenStorage stays responsible for token)
       await TokenStorage.saveToken(token.toString(), savedEmail);
 
@@ -90,6 +94,9 @@ class AuthService {
       final savedRole = (res['role'] ?? 'user').toString();
       final savedPhone = (res['phone_number'] ?? phoneNumber)?.toString();
 
+      // clear guest mode on successful registration
+      AuthGuard.setGuest(false);
+
       // Save token + canonical user id for token storage
       await TokenStorage.saveToken(token.toString(), savedEmail);
 

lib/screens/desktop_login_screen.dart

@@ -2,6 +2,7 @@
 
 import 'package:flutter/material.dart';
 import '../features/auth/services/auth_service.dart';
+import '../core/auth/auth_guard.dart';
 import 'home_desktop_screen.dart';
 import '../core/app_decoration.dart';
 
@@ -241,7 +242,17 @@ class _DesktopLoginScreenState extends State<DesktopLoginScreen> with SingleTick
                                       alignment: WrapAlignment.spaceBetween,
                                       children: [
                                         TextButton(onPressed: _openRegister, child: const Text("Don't have an account? Register")),
-                                        TextButton(onPressed: () {}, child: const Text('Contact support'))
+                                        TextButton(onPressed: () {}, child: const Text('Contact support')),
+                                        TextButton(
+                                          onPressed: () {
+                                            AuthGuard.setGuest(true);
+                                            Navigator.of(context).pushAndRemoveUntil(
+                                              MaterialPageRoute(builder: (_) => const HomeDesktopScreen(skipSidebarEntranceAnimation: true)),
+                                              (route) => false,
+                                            );
+                                          },
+                                          child: const Text('Continue as Guest'),
+                                        ),
                                       ],
                                     )
                                   ],

lib/screens/home_screen.dart

@@ -3,6 +3,7 @@ import 'dart:async';
 import 'dart:ui';
 import 'package:flutter/material.dart';
 import 'package:shared_preferences/shared_preferences.dart';
+import '../core/auth/auth_guard.dart';
 import 'package:cached_network_image/cached_network_image.dart';
 
 import '../features/events/models/event_models.dart';
@@ -448,7 +449,11 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
     final active = _selectedIndex == index;
 
     return GestureDetector(
-      onTap: () => setState(() => _selectedIndex = index),
+      onTap: () {
+        if (index == 2 && !AuthGuard.requireLogin(context, reason: 'Sign in to contribute events and earn rewards.')) return;
+        if (index == 3 && !AuthGuard.requireLogin(context, reason: 'Sign in to view your profile.')) return;
+        setState(() => _selectedIndex = index);
+      },
       behavior: HitTestBehavior.opaque,
       child: Padding(
         padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 4),

lib/screens/login_screen.dart

@@ -5,6 +5,7 @@ import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
 import 'package:video_player/video_player.dart';
 import '../features/auth/services/auth_service.dart';
+import '../core/auth/auth_guard.dart';
 import 'home_screen.dart';
 
 class LoginScreen extends StatefulWidget {
@@ -509,6 +510,30 @@ class _LoginScreenState extends State<LoginScreen> {
                             ],
                           ),
                         ),
+
+                        const SizedBox(height: 16),
+
+                        // Continue as Guest
+                        Center(
+                          child: GestureDetector(
+                            onTap: () {
+                              AuthGuard.setGuest(true);
+                              Navigator.of(context).pushAndRemoveUntil(
+                                MaterialPageRoute(builder: (_) => const HomeScreen()),
+                                (route) => false,
+                              );
+                            },
+                            child: const Text(
+                              'Continue as Guest',
+                              style: TextStyle(
+                                color: _textMuted,
+                                fontSize: 13,
+                                decoration: TextDecoration.underline,
+                                decorationColor: _textMuted,
+                              ),
+                            ),
+                          ),
+                        ),
                       ],
                     ),
                   ),