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

bmi_calculator/lib/example.dart

@@ -0,0 +1,12 @@
+import 'package:bmi_calculator/calculator/calculator_page.dart';
+import 'package:flutter/material.dart';
+
+// Example: BMI calculator entry screen.
+class Example extends StatelessWidget {
+  const Example({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return CalculatorPage(title: 'BMI CALCULATOR');
+  }
+}

bmi_calculator/lib/main.dart

@@ -1,7 +1,8 @@
-import 'package:bmi_calculator/calculator/calculator_page.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 
+import 'example.dart';
+
 void main() {
   WidgetsFlutterBinding.ensureInitialized();
 
@@ -20,7 +21,7 @@ class MyApp extends StatelessWidget {
   Widget build(BuildContext context) {
     return MaterialApp(
       title: 'BMI Calculator',
-      home: CalculatorPage(title: 'BMI CALCULATOR'),
+      home: const Example(),
     );
   }
 }