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