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

image_from_network/lib/example.dart

@@ -0,0 +1,27 @@
+import 'package:flutter/material.dart';
+
+// Example: load images (including animated GIFs) from a network URL.
+class Example extends StatelessWidget {
+  const Example({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text("Image from Network"),
+      ),
+      body: Container(
+          child: Column(
+        children: <Widget>[
+          // Load image from network
+          Image.network(
+              'https://github.com/nisrulz/flutter-examples/raw/develop/image_from_network/img/flutter_logo.png'),
+          // even loads gifs
+          // Gif image from Giphy, all copyrights are owned by Giphy
+          Image.network(
+              'https://github.com/nisrulz/flutter-examples/raw/develop/image_from_network/img/loop_anim.gif'),
+        ],
+      )),
+    );
+  }
+}

image_from_network/lib/main.dart

@@ -1,6 +1,10 @@
 import 'package:flutter/material.dart';
 
-void main() => runApp(MyApp());
+import 'example.dart';
+
+void main() {
+  runApp(const MyApp());
+}
 
 class MyApp extends StatelessWidget {
   const MyApp({super.key});
@@ -8,23 +12,8 @@ class MyApp extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
-      home: Scaffold(
-        appBar: AppBar(
-          title: Text("Image from Network"),
-        ),
-        body: Container(
-            child: Column(
-          children: <Widget>[
-            // Load image from network
-            Image.network(
-                'https://github.com/nisrulz/flutter-examples/raw/develop/image_from_network/img/flutter_logo.png'),
-            // even loads gifs
-            // Gif image from Giphy, all copyrights are owned by Giphy
-            Image.network(
-                'https://github.com/nisrulz/flutter-examples/raw/develop/image_from_network/img/loop_anim.gif'),
-          ],
-        )),
-      ),
+      title: "Image from Network",
+      home: const Example(),
     );
   }
 }