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_edittext/lib/example.dart

@@ -0,0 +1,47 @@
+import 'package:flutter/material.dart';
+
+// Example: a TextField that appends submitted lines to a result view.
+class Example extends StatefulWidget {
+  const Example({super.key});
+
+  @override
+  State<Example> createState() => _ExampleState();
+}
+
+class _ExampleState extends State<Example> {
+  String results = "";
+
+  final TextEditingController controller = TextEditingController();
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text("Using EditText"),
+        backgroundColor: Colors.red,
+      ),
+      body: Container(
+        padding: const EdgeInsets.all(10.0),
+        child: Center(
+          child: Column(
+            crossAxisAlignment: CrossAxisAlignment.stretch,
+            children: <Widget>[
+              TextField(
+                decoration:
+                    const InputDecoration(hintText: "Enter text here..."),
+                onSubmitted: (String str) {
+                  setState(() {
+                    results = "$results\n$str";
+                    controller.text = "";
+                  });
+                },
+                controller: controller,
+              ),
+              Text(results)
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}

using_edittext/lib/main.dart

@@ -1,51 +1,19 @@
 import 'package:flutter/material.dart';
 
+import 'example.dart';
+
 void main() {
-  runApp(MaterialApp(
-    home: MyEditText(),
-  ));
+  runApp(const MyApp());
 }
 
-class MyEditText extends StatefulWidget {
-  const MyEditText({super.key});
-
-  @override
-  MyEditTextState createState() => MyEditTextState();
-}
-
-class MyEditTextState extends State<MyEditText> {
-  String results = "";
-
-  final TextEditingController controller = TextEditingController();
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
 
   @override
   Widget build(BuildContext context) {
-    return Scaffold(
-      appBar: AppBar(
-        title: Text("Using EditText"),
-        backgroundColor: Colors.red,
-      ),
-      body: Container(
-        padding: const EdgeInsets.all(10.0),
-        child: Center(
-          child: Column(
-            crossAxisAlignment: CrossAxisAlignment.stretch,
-            children: <Widget>[
-              TextField(
-                decoration: InputDecoration(hintText: "Enter text here..."),
-                onSubmitted: (String str) {
-                  setState(() {
-                    results = "$results\n$str";
-                    controller.text = "";
-                  });
-                },
-                controller: controller,
-              ),
-              Text(results)
-            ],
-          ),
-        ),
-      ),
+    return MaterialApp(
+      title: "Using EditText",
+      home: const Example(),
     );
   }
 }