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

grid_layout/lib/example.dart

@@ -0,0 +1,22 @@
+// Example: A grid layout displaying social network icons in a 2-column grid.
+import 'package:flutter/material.dart';
+
+import 'gridview.dart';
+
+class Example extends StatelessWidget {
+  Example({super.key});
+
+  final MyGridView myGridView = MyGridView();
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        // Here we take the value from the MyHomePage object that was created by
+        // the App.build method, and use it to set our appbar title.
+        title: Text("GridView Example"),
+      ),
+      body: myGridView.build(),
+    );
+  }
+}

grid_layout/lib/main.dart

@@ -1,25 +1,17 @@
 import 'package:flutter/material.dart';
-import 'package:grid_layout/gridview.dart';
+
+import 'example.dart';
 
 void main() => runApp(MyApp());
 
 class MyApp extends StatelessWidget {
-  final MyGridView myGridView = MyGridView();
-
-  MyApp({super.key});
+  const MyApp({super.key});
 
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
       debugShowCheckedModeBanner: false,
-      home: Scaffold(
-        appBar: AppBar(
-          // Here we take the value from the MyHomePage object that was created by
-          // the App.build method, and use it to set our appbar title.
-          title: Text("GridView Example"),
-        ),
-        body: myGridView.build(),
-      ),
+      home: Example(),
     );
   }
 }