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

handling_routes/lib/example.dart

@@ -0,0 +1,56 @@
+// Example: A home screen with navigation to an About page via named routes.
+import 'package:flutter/material.dart';
+
+class Example extends StatelessWidget {
+  const Example({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      // AppBar
+      appBar: AppBar(
+        // Title
+        title: Text("Home Page"),
+        // App Bar background color
+        backgroundColor: Colors.red,
+      ),
+      // Body
+      body: Container(
+        // Center the content
+        child: Center(
+          child: Column(
+            // Center content in the column
+            mainAxisAlignment: MainAxisAlignment.center,
+            // add children to the column
+            children: <Widget>[
+              // Text
+              Text(
+                "Home Page\nClick on below icon to goto About Page",
+                // Setting the style for the Text
+                style: TextStyle(
+                  fontSize: 20.0,
+                ),
+                // Set text alignment to center
+                textAlign: TextAlign.center,
+              ),
+              // Icon Button
+              IconButton(
+                icon: Icon(
+                  Icons.info,
+                  color: Colors.blue,
+                ),
+                // Execute when pressed
+                onPressed: () {
+                  // use the navigator to goto a named route
+                  Navigator.of(context).pushNamed('/about');
+                },
+                // Setting the size of icon
+                iconSize: 80.0,
+              )
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}