refactor: split app wrapper from focused example code (multi-file apps)

Convert 17 multi-file apps to the validated pattern: lib/main.dart keeps
only the MaterialApp bootstrap; a new lib/example.dart holds the full entry
screen and focused example code, importing existing supporting files
(screens/, tabs/, services/, models/, widgets/, utils/).

Apps: analytics_integration, animation_example, biometrics, bottom_sheet,
custom_home_drawer, google_signin, grid_layout, handling_routes,
image_editor, scan_qr_code, statless_counter_app, using_bottom_nav_bar,
using_custom_fonts, using_listview, using_listwheelscrollview,
using_platform_adaptive, view_pdf_file

scan_qr_code/lib/main.dart

@@ -1,8 +1,5 @@
-
 import 'package:flutter/material.dart';
-import 'package:mobile_scanner/mobile_scanner.dart';
-import 'package:scan_qr_code/utils/qr_code_scanner.dart';
-import 'package:scan_qr_code/utils/scanner_box_border_painter.dart';
+import 'package:scan_qr_code/example.dart';
 
 void main() => runApp(const MyApp());
 
@@ -13,82 +10,7 @@ class MyApp extends StatelessWidget {
   Widget build(BuildContext context) {
     return const MaterialApp(
       title: 'Code Scanner',
-      home: MyHomePage(title: 'Code Scanner Demo'),
-    );
-  }
-}
-
-class MyHomePage extends StatefulWidget {
-  const MyHomePage({super.key, required this.title});
-
-  final String title;
-
-  @override
-  State<MyHomePage> createState() => _MyHomePageState();
-}
-
-class _MyHomePageState extends State<MyHomePage> {
-  final MobileScannerController _mobileScannerController =
-      MobileScannerController();
-  late void Function(BarcodeCapture) _onDetect;
-  bool _isScanning = true;
-
-  @override
-  void initState() {
-    super.initState();
-    _onDetect = (BarcodeCapture capture) {
-      _mobileScannerController.stop();
-      showDialog(
-          context: context,
-          builder: (BuildContext context) {
-            return AlertDialog(
-              title: const Text('Code info'),
-              content: Text(
-                  capture.barcodes.first.rawValue?.trim() ?? 'No data found',
-                  textAlign: TextAlign.center),
-              actions: [
-                TextButton(
-                    onPressed: () => Navigator.pop(context),
-                    child: const Text('OK'))
-              ],
-            );
-          }).then((value) {
-        _mobileScannerController.start();
-        setState(() {
-          _isScanning = true;
-        });
-      });
-
-      setState(() {
-        _isScanning = false;
-      });
-    };
-  }
-
-  @override
-  Widget build(BuildContext context) {
-    return Scaffold(
-      appBar: AppBar(title: Text(widget.title)),
-      body: Center(
-        child: SizedBox.square(
-          dimension: MediaQuery.of(context).size.width - 48.0,
-          child: Padding(
-            padding: const EdgeInsets.all(
-                2.5), // Required otherwise side edges hides under outside padding
-            child: CustomPaint(
-              foregroundPainter: ScannerBoxBorderPainter(
-                  borderColor: _isScanning ? Colors.black : Colors.green),
-              child: Padding(
-                padding: const EdgeInsets.all(
-                    2.5), // Required otherwise scanner hides under side edges
-                child: QRCodeScanner(
-                    mobileScannerController: _mobileScannerController,
-                    onDetect: _onDetect),
-              ),
-            ),
-          ),
-        ),
-      ),
+      home: Example(),
     );
   }
 }