feat: Phase 1 critical gaps — gamification API, Razorpay checkout, Google OAuth, notifications

- Fix gamification endpoints to use Node.js server (app.eventifyplus.com)
- Replace 6 mock gamification methods with real API calls (dashboard, leaderboard, shop, redeem, submit)
- Add booking models, service, payment service (Razorpay), checkout provider
- Add 3-step CheckoutScreen with Razorpay native modal integration
- Add Google OAuth login (Flutter + Django backend)
- Add full notifications system (Django model + 3 endpoints + Flutter UI)
- Register CheckoutProvider, NotificationProvider in main.dart MultiProvider
- Wire notification bell in HomeScreen app bar
- Add razorpay_flutter ^1.3.7 and google_sign_in ^6.2.2 packages

lib/screens/login_screen.dart

@@ -5,9 +5,13 @@ import '../core/utils/error_utils.dart';
 import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
 import 'package:video_player/video_player.dart';
+import 'package:provider/provider.dart';
 import '../features/auth/services/auth_service.dart';
+import '../features/auth/providers/auth_provider.dart';
 import '../core/auth/auth_guard.dart';
 import 'home_screen.dart';
+import 'responsive_layout.dart';
+import 'home_desktop_screen.dart';
 
 class LoginScreen extends StatefulWidget {
   const LoginScreen({Key? key}) : super(key: key);
@@ -124,6 +128,27 @@ class _LoginScreenState extends State<LoginScreen> {
     );
   }
 
+  Future<void> _performGoogleLogin() async {
+    try {
+      setState(() => _loading = true);
+      await Provider.of<AuthProvider>(context, listen: false).googleLogin();
+      if (mounted) {
+        Navigator.pushReplacement(
+          context,
+          MaterialPageRoute(builder: (_) => ResponsiveLayout(mobile: HomeScreen(), desktop: const HomeDesktopScreen())),
+        );
+      }
+    } catch (e) {
+      if (mounted) {
+        ScaffoldMessenger.of(context).showSnackBar(
+          SnackBar(content: Text(userFriendlyError(e))),
+        );
+      }
+    } finally {
+      if (mounted) setState(() => _loading = false);
+    }
+  }
+
   /// Glassmorphism pill-shaped input decoration
   InputDecoration _glassInputDecoration({
     required String hint,
@@ -474,7 +499,7 @@ class _LoginScreenState extends State<LoginScreen> {
                                   color: Color(0xFF4285F4),
                                 ),
                               ),
-                              onTap: _showComingSoon,
+                              onTap: _performGoogleLogin,
                             ),
                             const SizedBox(width: 12),
                             _socialButton(