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/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,
+                              ),
+                            ),
+                          ),
+                        ),
                       ],
                     ),
                   ),