refactor: split app wrapper from focused example code (complex apps)

Convert the final 12 multi-screen apps to the validated pattern:
lib/main.dart keeps only the MaterialApp bootstrap (plus firebase init where
that is genuine bootstrap), and a new lib/example.dart holds the entry screen
and focused example code, importing existing supporting files. firebase_crash
keeps its runZonedGuarded bootstrap in main.dart and extracts only CrashApp.

Apps: bmi_calculator, covid19_mobile_app, expense_planner,
firebase_crash_reporting, firebase_google_authentication, lunch_app,
navigation_drawer, news_memes_app, save_data_locally_with_sqlite,
todo_list_using_provider, unit_testing, using_firebase_db

firebase_google_authentication/lib/example.dart

@@ -0,0 +1,13 @@
+// Example: Google sign-in flow with login, signup, and an authenticated info page.
+
+import 'package:firebase_google_authentication/Screens/HomePage.dart';
+import 'package:flutter/material.dart';
+
+class Example extends StatelessWidget {
+  const Example({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const HomePage();
+  }
+}

firebase_google_authentication/lib/main.dart

@@ -1,13 +1,13 @@
 import 'package:firebase_core/firebase_core.dart';
-import 'package:firebase_google_authentication/Screens/HomePage.dart';
 import 'package:firebase_google_authentication/Services/google_auth.dart';
+import 'package:firebase_google_authentication/example.dart';
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 
-void main() async{
+void main() async {
   WidgetsFlutterBinding.ensureInitialized();
   await Firebase.initializeApp();
-  runApp(MyApp());
+  runApp(const MyApp());
 }
 
 class MyApp extends StatelessWidget {
@@ -19,7 +19,7 @@ class MyApp extends StatelessWidget {
       create: (context) => GoogleSignInProvider(),
       child: MaterialApp(
         debugShowCheckedModeBanner: false,
-        home: HomePage(),
+        home: const Example(),
       ),
     );
   }