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

load_local_image/lib/example.dart

@@ -0,0 +1,28 @@
+import 'package:flutter/material.dart';
+
+// Example: load an image from the app's assets and use it as a background.
+class Example extends StatelessWidget {
+  const Example({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+        appBar: AppBar(
+          title: const Text("Load local image"),
+        ),
+        body: Container(
+          decoration: const BoxDecoration(
+              image: DecorationImage(
+                  // Load image from assets
+                  image: AssetImage('data_repo/img/bg1.jpg'),
+                  // Make the image cover the whole area
+                  fit: BoxFit.cover)),
+          child: const Center(
+            child: Text(
+              "Hello World!",
+              style: TextStyle(color: Colors.white),
+            ),
+          ),
+        ));
+  }
+}

load_local_image/lib/main.dart

@@ -1,9 +1,9 @@
 import 'package:flutter/material.dart';
 
+import 'example.dart';
+
 void main() {
-  runApp(MaterialApp(
-    home: MyApp(),
-  ));
+  runApp(const MyApp());
 }
 
 class MyApp extends StatelessWidget {
@@ -11,23 +11,9 @@ class MyApp extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return Scaffold(
-        appBar: AppBar(
-          title: Text("Load local image"),
-        ),
-        body: Container(
-          decoration: BoxDecoration(
-              image: DecorationImage(
-                  // Load image from assets
-                  image: AssetImage('data_repo/img/bg1.jpg'),
-                  // Make the image cover the whole area
-                  fit: BoxFit.cover)),
-          child: Center(
-            child: Text(
-              "Hello World!",
-              style: TextStyle(color: Colors.white),
-            ),
-          ),
-        ));
+    return MaterialApp(
+      title: "Load local image",
+      home: const Example(),
+    );
   }
 }