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

navigation_drawer/lib/example.dart

@@ -0,0 +1,13 @@
+// Example: A home screen with a navigation drawer to switch between settings and account screens.
+
+import 'package:flutter/material.dart';
+import 'package:navigation_drawer/screens/home.dart';
+
+class Example extends StatelessWidget {
+  const Example({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const HomeScreen();
+  }
+}

navigation_drawer/lib/main.dart

@@ -1,16 +1,25 @@
 import 'package:flutter/material.dart';
+import 'package:navigation_drawer/example.dart';
 import 'package:navigation_drawer/screens/account.dart';
-import 'package:navigation_drawer/screens/home.dart';
 import 'package:navigation_drawer/screens/settings.dart';
 
 void main() {
-  runApp(MaterialApp(
-    debugShowCheckedModeBanner: false,
-    home: HomeScreen(), // route for home is '/' implicitly
-    routes: <String, WidgetBuilder>{
-      // define the routes
-      SettingsScreen.routeName: (BuildContext context) => SettingsScreen(),
-      AccountScreen.routeName: (BuildContext context) => AccountScreen(),
-    },
-  ));
+  runApp(const MyApp());
+}
+
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      debugShowCheckedModeBanner: false,
+      home: const Example(), // route for home is '/' implicitly
+      routes: <String, WidgetBuilder>{
+        // define the routes
+        SettingsScreen.routeName: (BuildContext context) => SettingsScreen(),
+        AccountScreen.routeName: (BuildContext context) => AccountScreen(),
+      },
+    );
+  }
 }