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

animation_example/lib/example.dart

@@ -1,14 +1,15 @@
+// Example: A compass-like animation demo where tapping rotates a pen image
+// and points it toward a direction (north/west/south/east).
 import 'package:flutter/material.dart';
 
-class HomePage extends StatefulWidget {
-  const HomePage({super.key});
+class Example extends StatefulWidget {
+  const Example({super.key});
 
   @override
-  _HomePageState createState() => _HomePageState();
+  _ExampleState createState() => _ExampleState();
 }
 
-class _HomePageState extends State<HomePage>
-    with SingleTickerProviderStateMixin {
+class _ExampleState extends State<Example> with SingleTickerProviderStateMixin {
   late AnimationController controller;
   var target = 0.0;
   final Map<double, String> data = {
@@ -54,8 +55,7 @@ class _HomePageState extends State<HomePage>
                 child: TextButton(
                     onPressed: () {},
                     style: ButtonStyle(
-                        backgroundColor:
-                            WidgetStateProperty.all(Colors.green)),
+                        backgroundColor: WidgetStateProperty.all(Colors.green)),
                     child: const Text(
                       "North",
                       style: TextStyle(
@@ -104,8 +104,7 @@ class _HomePageState extends State<HomePage>
                 child: TextButton(
                     onPressed: () {},
                     style: ButtonStyle(
-                        backgroundColor:
-                            WidgetStateProperty.all(Colors.green)),
+                        backgroundColor: WidgetStateProperty.all(Colors.green)),
                     child: const Text(
                       "North",
                       style: TextStyle(

animation_example/lib/main.dart

@@ -1,6 +1,6 @@
 import 'package:flutter/material.dart';
 
-import 'home_page.dart';
+import 'example.dart';
 
 void main() {
   runApp(const MyApp());
@@ -15,10 +15,9 @@ class MyApp extends StatelessWidget {
     return MaterialApp(
       title: 'Pen Assignement',
       theme: ThemeData(
-
         primarySwatch: Colors.blue,
       ),
-      home:HomePage(),
+      home: const Example(),
     );
   }
 }