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

using_bottom_nav_bar/lib/example.dart

@@ -0,0 +1,75 @@
+// Example: Using bottom navigation bar with tabs
+import 'package:flutter/material.dart';
+import 'package:using_bottom_nav_bar/tabs/first.dart';
+import 'package:using_bottom_nav_bar/tabs/second.dart';
+import 'package:using_bottom_nav_bar/tabs/third.dart';
+
+class Example extends StatefulWidget {
+  const Example({super.key});
+
+  @override
+  ExampleState createState() => ExampleState();
+}
+
+// SingleTickerProviderStateMixin is used for animation
+class ExampleState extends State<Example> with SingleTickerProviderStateMixin {
+  // Create a tab controller
+  late TabController controller;
+
+  @override
+  void initState() {
+    super.initState();
+
+    // Initialize the Tab Controller
+    controller = TabController(length: 3, vsync: this);
+  }
+
+  @override
+  void dispose() {
+    // Dispose of the Tab Controller
+    controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      // Appbar
+      appBar: AppBar(
+        // Title
+        title: Text("Using Bottom Navigation Bar"),
+        // Set the background color of the App Bar
+        backgroundColor: Colors.blue,
+      ),
+      // Set the TabBar view as the body of the Scaffold
+      body: TabBarView(
+        // Add tabs as widgets
+        controller: controller,
+        // Add tabs as widgets
+        children: <Widget>[FirstTab(), SecondTab(), ThirdTab()],
+      ),
+      // Set the bottom navigation bar
+      bottomNavigationBar: Material(
+        // set the color of the bottom navigation bar
+        color: Colors.blue,
+        // set the tab bar as the child of bottom navigation bar
+        child: TabBar(
+          tabs: <Tab>[
+            Tab(
+              // set icon to the tab
+              icon: Icon(Icons.favorite),
+            ),
+            Tab(
+              icon: Icon(Icons.adb),
+            ),
+            Tab(
+              icon: Icon(Icons.airport_shuttle),
+            ),
+          ],
+          // setup the controller
+          controller: controller,
+        ),
+      ),
+    );
+  }
+}