add auth skeleton

wien_talks/wien_talks_flutter/lib/helper/auth_service.dart

@@ -0,0 +1,30 @@
+import 'package:google_sign_in/google_sign_in.dart';
+
+class AuthService {
+  static final _google = GoogleSignIn.instance;
+
+  static AuthService? _instance;
+
+  AuthService._() {
+    _google.initialize();
+  }
+  factory AuthService() {
+    if (_instance != null) return _instance!;
+    _instance = AuthService._();
+
+    return _instance!;
+  }
+
+  static Stream<GoogleSignInAuthenticationEvent?> get onUserChanged =>
+      _google.authenticationEvents;
+
+  static Future<GoogleSignInAccount?> signIn() async {
+    try {
+      return await _google.authenticate();
+    } catch (_) {
+      return null;
+    }
+  }
+
+  static Future<void> signOut() => _google.disconnect();
+}

wien_talks/wien_talks_flutter/lib/helper/go_router.dart

@@ -1,13 +1,31 @@
+import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 import 'package:wien_talks_flutter/create_event_screen.dart';
-import 'package:wien_talks_flutter/home_screen.dart';
+import 'package:wien_talks_flutter/helper/auth_service.dart';
+import 'package:wien_talks_flutter/login_page.dart';
+import 'package:wien_talks_flutter/news_screen.dart';
 
 final router = GoRouter(
+  redirect: (context, state) {
+    final loggedIn = AuthService.user != null;
+    final atLogin = state.matchedLocation == '/login';
+    if (!loggedIn && !atLogin) return '/login';
+    if (loggedIn && atLogin) return '/';
+    return null;
+  },
+  refreshListenable: AuthChangeNotifier(),
   routes: [
+    GoRoute(path: '/login', builder: (c, s) => const LoginScreen()),
+    GoRoute(path: '/', builder: (c, s) => NewsScreen()),
     GoRoute(
-      path: '/',
-      builder: (context, state) => HomeScreen(),
-    ),
-    GoRoute(path: '/create_event', name: 'create_event', builder: (context, state) => CreateEventScreen()),
+        path: '/create_event',
+        name: 'create_event',
+        builder: (c, s) => CreateEventScreen()),
   ],
 );
+
+class AuthChangeNotifier extends ChangeNotifier {
+  AuthChangeNotifier() {
+    AuthService.onUserChanged.listen((_) => notifyListeners());
+  }
+}

wien_talks/wien_talks_flutter/lib/login_page.dart

@@ -0,0 +1,50 @@
+import 'package:flutter/material.dart';
+import 'package:google_fonts/google_fonts.dart';
+import 'package:wien_talks_flutter/helper/auth_service.dart';
+
+class LoginScreen extends StatelessWidget {
+  const LoginScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: Container(
+        decoration: const BoxDecoration(
+          gradient: LinearGradient(
+            colors: [Color(0xff2193b0), Color(0xff6dd5ed)],
+            begin: Alignment.topLeft,
+            end: Alignment.bottomRight,
+          ),
+        ),
+        alignment: Alignment.center,
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Text('Wien Talks',
+                style: GoogleFonts.poppins(
+                    fontSize: 42,
+                    fontWeight: FontWeight.bold,
+                    color: Colors.white)),
+            const SizedBox(height: 60),
+            FilledButton.icon(
+              onPressed: () async => await AuthService.signIn(),
+              style: FilledButton.styleFrom(
+                backgroundColor: Colors.white,
+                foregroundColor: Colors.black87,
+                padding:
+                    const EdgeInsets.symmetric(horizontal: 24, vertical: 12),
+                shape: RoundedRectangleBorder(
+                    borderRadius: BorderRadius.circular(30)),
+                elevation: 6,
+              ),
+              icon: Icon(
+                Icons.lock,
+              ),
+              label: const Text('Sign in with Google'),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}