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

custom_home_drawer/lib/main.dart

@@ -1,4 +1,4 @@
-import 'package:custom_home_drawer/screen/home_screen.dart';
+import 'package:custom_home_drawer/example.dart';
 import 'package:flutter/material.dart';
 
 void main() {
@@ -15,7 +15,7 @@ class MyApp extends StatelessWidget {
       theme: ThemeData(
         primarySwatch: Colors.blue,
       ),
-      home: const HomeScreen(),
+      home: const Example(),
     );
   }
 }