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

statless_counter_app/lib/example.dart

@@ -0,0 +1,42 @@
+// Example: Stateless widget counter using MobX
+import 'package:flutter/material.dart';
+import 'package:flutter_mobx/flutter_mobx.dart';
+
+import 'counter.dart'; // Import the Counter
+
+final counter = Counter(); // Instantiate the store
+
+class Example extends StatelessWidget {
+  const Example({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('MobX Stateless Widget Counter'),
+      ),
+      body: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: <Widget>[
+            Text(
+              'You have pushed the button this many times:',
+            ),
+            // Wrapping in the Observer will automatically re-render on changes to counter.value
+            Observer(
+              builder: (_) => Text(
+                '${counter.value}',
+                style: Theme.of(context).textTheme.headlineSmall,
+              ),
+            ),
+          ],
+        ),
+      ),
+      floatingActionButton: FloatingActionButton(
+        onPressed: counter.increment,
+        tooltip: 'Increment',
+        child: Icon(Icons.add),
+      ),
+    );
+  }
+}

statless_counter_app/lib/main.dart

@@ -1,11 +1,8 @@
 import 'package:flutter/material.dart';
-import 'package:flutter_mobx/flutter_mobx.dart';
 
-import 'counter.dart'; // Import the Counter
+import 'example.dart';
 
-final counter = Counter(); // Instantiate the store
-
-void main() => runApp(MyApp());
+void main() => runApp(const MyApp());
 
 class MyApp extends StatelessWidget {
   const MyApp({super.key});
@@ -19,42 +16,7 @@ class MyApp extends StatelessWidget {
       theme: ThemeData(
         primarySwatch: Colors.blue,
       ),
-      home: const MyHomePage(),
+      home: const Example(),
     );
   }
 }
-
-class MyHomePage extends StatelessWidget {
-  const MyHomePage({super.key});
-
-  @override
-  Widget build(BuildContext context) {
-    return Scaffold(
-      appBar: AppBar(
-        title: Text('MobX Stateless Widget Counter'),
-      ),
-      body: Center(
-        child: Column(
-          mainAxisAlignment: MainAxisAlignment.center,
-          children: <Widget>[
-            Text(
-              'You have pushed the button this many times:',
-            ),
-            // Wrapping in the Observer will automatically re-render on changes to counter.value
-            Observer(
-              builder: (_) => Text(
-                '${counter.value}',
-                style: Theme.of(context).textTheme.headlineSmall,
-              ),
-            ),
-          ],
-        ),
-      ),
-      floatingActionButton: FloatingActionButton(
-        onPressed: counter.increment,
-        tooltip: 'Increment',
-        child: Icon(Icons.add),
-      ),
-    );
-  }
-}