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_crash_reporting/lib/example.dart

@@ -0,0 +1,31 @@
+import 'package:firebase_crashlytics/firebase_crashlytics.dart';
+import 'package:flutter/material.dart';
+
+// Example: Two buttons to test Crashlytics custom logging and crash reporting.
+class Example extends StatelessWidget {
+  const Example({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Center(
+      child: Column(
+        mainAxisAlignment: MainAxisAlignment.center,
+        children: [
+          ElevatedButton(
+              onPressed: () {
+                //custom Crashlytics log message
+                FirebaseCrashlytics.instance.log("It's a bug");
+              },
+              child: Text("Custom Log")),
+          const SizedBox(height: 10),
+          ElevatedButton(
+            child: Text('Crash the app'),
+            onPressed: () {
+              FirebaseCrashlytics.instance.crash();
+            },
+          ),
+        ],
+      ),
+    );
+  }
+}

firebase_crash_reporting/lib/main.dart

@@ -4,6 +4,8 @@ import 'package:firebase_core/firebase_core.dart';
 import 'package:firebase_crashlytics/firebase_crashlytics.dart';
 import 'package:flutter/material.dart';
 
+import 'example.dart';
+
 void main() {
   WidgetsFlutterBinding.ensureInitialized();
   runZonedGuarded(() {
@@ -17,12 +19,12 @@ void main() {
 class App extends StatelessWidget {
   const App({super.key});
 
-
   //initialise firebase and crashlytics
   Future<void> _initializeFirebase() async {
     await Firebase.initializeApp();
     await FirebaseCrashlytics.instance.setCrashlyticsCollectionEnabled(true);
   }
+
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
@@ -40,7 +42,7 @@ class App extends StatelessWidget {
             }
             //firebase and crashlytics initialise complete
             if (snapshot.connectionState == ConnectionState.done) {
-              return CrashApp();
+              return const Example();
             }
             return Center(
               child: Column(
@@ -56,31 +58,3 @@ class App extends StatelessWidget {
     );
   }
 }
-
-class CrashApp extends StatelessWidget {
-  const CrashApp({super.key});
-
-  @override
-  Widget build(BuildContext context) {
-    return Center(
-      child: Column(
-        mainAxisAlignment: MainAxisAlignment.center,
-        children: [
-          ElevatedButton(
-              onPressed: () {
-                //custom Crashlytics log message
-                FirebaseCrashlytics.instance.log("It's a bug");
-              },
-              child: Text("Custom Log")),
-          const SizedBox(height: 10),
-          ElevatedButton(
-            child: Text('Crash the app'),
-            onPressed: () {
-              FirebaseCrashlytics.instance.crash();
-            },
-          ),
-        ],
-      ),
-    );
-  }
-}