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),
+          ],
+        ),
+      ),
+    );
+  }
+}