refactor: split app wrapper from focused example code (single-file apps)

Convert 20 more single-file apps to the validated pattern: lib/main.dart
holds only the runApp + MaterialApp bootstrap, while a new lib/example.dart
holds the full screen (Scaffold, AppBar) and the focused example code.

Apps: enabling_splash_screen, getx_counter_app, image_from_network,
infinite_list, load_local_image, load_local_json, persist_key_value,
push_notifications, sliver_app_bar_example, stateless_widgets, tic_tac_toe,
tip_calculator, using_alert_dialog, using_edittext, using_expansionpanel,
using_http_get, using_interactiveviewer, using_snackbar, using_stepper,
using_theme

using_alert_dialog/lib/example.dart

@@ -0,0 +1,39 @@
+import 'package:flutter/material.dart';
+
+// Example: show an AlertDialog when the button is pressed.
+class Example extends StatefulWidget {
+  const Example({super.key});
+
+  @override
+  State<Example> createState() => _ExampleState();
+}
+
+class _ExampleState extends State<Example> {
+  // Generate dialog
+  AlertDialog dialog = AlertDialog(
+      content: const Text(
+    "Hello World!",
+    style: TextStyle(fontSize: 30.0),
+  ));
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+        appBar: AppBar(
+          title: const Text("Using Alert Dialog"),
+        ),
+        body: Container(
+          child: Center(
+            child: ElevatedButton(
+                child: const Text("Hit to alert!"),
+                // On press of the button
+                onPressed: () {
+                  // Show dialog
+                  showDialog(
+                      context: context,
+                      builder: (BuildContext context) => dialog);
+                }),
+          ),
+        ));
+  }
+}

using_alert_dialog/lib/main.dart

@@ -1,42 +1,19 @@
 import 'package:flutter/material.dart';
 
+import 'example.dart';
+
 void main() {
-  runApp(MaterialApp(
-    home: MyHome(),
-  ));
+  runApp(const MyApp());
 }
 
-class MyHome extends StatefulWidget {
-  const MyHome({super.key});
-
-  @override
-  MyHomeState createState() => MyHomeState();
-}
-
-class MyHomeState extends State<MyHome> {
-  // Generate dialog
-  AlertDialog dialog = AlertDialog(
-      content: Text(
-    "Hello World!",
-    style: TextStyle(fontSize: 30.0),
-  ));
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
 
   @override
   Widget build(BuildContext context) {
-    return Scaffold(
-        appBar: AppBar(
-          title: Text("Using Alert Dialog"),
-        ),
-        body: Container(
-          child: Center(
-            child: ElevatedButton(
-                child: Text("Hit to alert!"),
-                // On press of the button
-                onPressed: () {
-                  // Show dialog
-                  showDialog(context: context, builder: (BuildContext context) => dialog);
-                }),
-          ),
-        ));
+    return MaterialApp(
+      title: "Using Alert Dialog",
+      home: const Example(),
+    );
   }
 }