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

expense_planner/lib/main.dart

@@ -1,11 +1,8 @@
 import 'package:flutter/material.dart';
 
-import './widgets/new_transaction.dart';
-import './widgets/transaction_list.dart';
-import './widgets/chart.dart';
-import './models/transaction.dart';
+import 'example.dart';
 
-void main() => runApp(MyApp());
+void main() => runApp(const MyApp());
 
 class MyApp extends StatelessWidget {
   const MyApp({super.key});
@@ -34,92 +31,7 @@ class MyApp extends StatelessWidget {
               fontWeight: FontWeight.bold,
             ),
           )),
-      home: MyHomePage(),
-    );
-  }
-}
-
-class MyHomePage extends StatefulWidget {
-  const MyHomePage({super.key});
-
-  @override
-  _MyHomePageState createState() => _MyHomePageState();
-}
-
-class _MyHomePageState extends State<MyHomePage> {
-  final List<Transaction> _userTransactions = [];
-
-  List<Transaction> get _recentTransactions {
-    return _userTransactions.where((tx) {
-      return tx.date.isAfter(
-        DateTime.now().subtract(
-          Duration(days: 7),
-        ),
-      );
-    }).toList();
-  }
-
-  void _addNewTransaction(
-      String txTitle, double txAmount, DateTime chosenDate) {
-    final newTx = Transaction(
-      title: txTitle,
-      amount: txAmount,
-      date: chosenDate,
-      id: DateTime.now().toString(),
-    );
-
-    setState(() {
-      _userTransactions.add(newTx);
-    });
-  }
-
-  void _startAddNewTransaction(BuildContext ctx) {
-    showModalBottomSheet(
-      context: ctx,
-      builder: (_) {
-        return GestureDetector(
-          onTap: () {},
-          behavior: HitTestBehavior.opaque,
-          child: NewTransaction(_addNewTransaction),
-        );
-      },
-    );
-  }
-
-  void _deleteTransaction(String id) {
-    setState(() {
-      _userTransactions.removeWhere((tx) => tx.id == id);
-    });
-  }
-
-  @override
-  Widget build(BuildContext context) {
-    return Scaffold(
-      appBar: AppBar(
-        title: Text(
-          'Personal Expenses',
-        ),
-        actions: <Widget>[
-          IconButton(
-            icon: Icon(Icons.add),
-            onPressed: () => _startAddNewTransaction(context),
-          ),
-        ],
-      ),
-      body: SingleChildScrollView(
-        child: Column(
-          crossAxisAlignment: CrossAxisAlignment.stretch,
-          children: <Widget>[
-            Chart(_recentTransactions),
-            TransactionList(_userTransactions, _deleteTransaction),
-          ],
-        ),
-      ),
-      floatingActionButtonLocation: FloatingActionButtonLocation.centerFloat,
-      floatingActionButton: FloatingActionButton(
-        child: Icon(Icons.add),
-        onPressed: () => _startAddNewTransaction(context),
-      ),
+      home: const Example(),
     );
   }
 }