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

todo_list_using_provider/lib/example.dart

@@ -0,0 +1,18 @@
+// Example: Todo list using provider
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+import 'notifiers/todo_list.dart';
+import 'views/home.dart';
+
+class Example extends StatelessWidget {
+  const Example({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return ChangeNotifierProvider<TodoList>(
+      create: (context) => TodoList(),
+      child: const MyHomePage(title: 'Flutter Demo Home Page'),
+    );
+  }
+}

todo_list_using_provider/lib/main.dart

@@ -1,8 +1,5 @@
 import 'package:flutter/material.dart';
-import 'package:provider/provider.dart';
-
-import 'notifiers/todo_list.dart';
-import 'views/home.dart';
+import 'example.dart';
 
 void main() {
   runApp(MyApp());
@@ -20,10 +17,7 @@ class MyApp extends StatelessWidget {
         primarySwatch: Colors.blue,
         visualDensity: VisualDensity.adaptivePlatformDensity,
       ),
-      home: ChangeNotifierProvider<TodoList>(
-        create: (context) => TodoList(),
-        child: MyHomePage(title: 'Flutter Demo Home Page'),
-      ),
+      home: const Example(),
     );
   }
-}
+}