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

covid19_mobile_app/lib/example.dart

@@ -0,0 +1,12 @@
+// Example: World Wide COVID-19 cases with daily charts
+import 'package:flutter/material.dart';
+import 'screens/home.dart';
+
+class Example extends StatelessWidget {
+  const Example({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Home();
+  }
+}