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_json/lib/example.dart

@@ -0,0 +1,57 @@
+import 'dart:convert';
+
+import 'package:flutter/material.dart';
+
+// Example: load and decode a local JSON asset, then list its rows.
+class Example extends StatefulWidget {
+  const Example({super.key});
+
+  @override
+  State<Example> createState() => _ExampleState();
+}
+
+class _ExampleState extends State<Example> {
+  late List data;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+        appBar: AppBar(
+          title: const Text("Load local JSON file"),
+        ),
+        body: Container(
+          child: Center(
+            // Use future builder and DefaultAssetBundle to load the local JSON file
+            child: FutureBuilder(
+                future: DefaultAssetBundle.of(context)
+                    .loadString('data_repo/starwars_data.json'),
+                builder: (context, snapshot) {
+                  // Decode the JSON
+                  var newData = json.decode(snapshot.data.toString());
+
+                  return ListView.builder(
+                    // Build the ListView
+                    itemBuilder: (BuildContext context, int index) {
+                      return Card(
+                        child: Column(
+                          crossAxisAlignment: CrossAxisAlignment.stretch,
+                          children: <Widget>[
+                            Text("Name: ${newData[index]['name']}"),
+                            Text("Height: ${newData[index]['height']}"),
+                            Text("Mass: ${newData[index]['mass']}"),
+                            Text("Hair Color: ${newData[index]['hair_color']}"),
+                            Text("Skin Color: ${newData[index]['skin_color']}"),
+                            Text("Eye Color: ${newData[index]['eye_color']}"),
+                            Text("Birth Year: ${newData[index]['birth_year']}"),
+                            Text("Gender: ${newData[index]['gender']}")
+                          ],
+                        ),
+                      );
+                    },
+                    itemCount: newData == null ? 0 : newData.length,
+                  );
+                }),
+          ),
+        ));
+  }
+}

load_local_json/lib/main.dart

@@ -1,62 +1,19 @@
-import 'dart:convert';
-
 import 'package:flutter/material.dart';
 
+import 'example.dart';
+
 void main() {
-  runApp(MaterialApp(
-    home: MyApp(),
-  ));
+  runApp(const MyApp());
 }
 
-class MyApp extends StatefulWidget {
+class MyApp extends StatelessWidget {
   const MyApp({super.key});
 
-  @override
-  MyAppState createState() => MyAppState();
-}
-
-class MyAppState extends State<MyApp> {
-  late List data;
-
   @override
   Widget build(BuildContext context) {
-    return Scaffold(
-        appBar: AppBar(
-          title: Text("Load local JSON file"),
-        ),
-        body: Container(
-          child: Center(
-            // Use future builder and DefaultAssetBundle to load the local JSON file
-            child: FutureBuilder(
-                future: DefaultAssetBundle.of(context)
-                    .loadString('data_repo/starwars_data.json'),
-                builder: (context, snapshot) {
-                  // Decode the JSON
-                  var newData = json.decode(snapshot.data.toString());
-
-                  return ListView.builder(
-                    // Build the ListView
-                    itemBuilder: (BuildContext context, int index) {
-                      return Card(
-                        child: Column(
-                          crossAxisAlignment: CrossAxisAlignment.stretch,
-                          children: <Widget>[
-                            Text("Name: ${newData[index]['name']}"),
-                            Text("Height: ${newData[index]['height']}"),
-                            Text("Mass: ${newData[index]['mass']}"),
-                            Text("Hair Color: ${newData[index]['hair_color']}"),
-                            Text("Skin Color: ${newData[index]['skin_color']}"),
-                            Text("Eye Color: ${newData[index]['eye_color']}"),
-                            Text("Birth Year: ${newData[index]['birth_year']}"),
-                            Text("Gender: ${newData[index]['gender']}")
-                          ],
-                        ),
-                      );
-                    },
-                    itemCount: newData == null ? 0 : newData.length,
-                  );
-                }),
-          ),
-        ));
+    return MaterialApp(
+      title: "Load local JSON file",
+      home: const Example(),
+    );
   }
 }