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();
+            },
+          ),
+        ],
+      ),
+    );
+  }
+}