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/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),