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

enabling_splash_screen/lib/example.dart

@@ -0,0 +1,18 @@
+import 'package:flutter/material.dart';
+
+// Example: how to enable a splash screen and show a simple message.
+class Example extends StatelessWidget {
+  const Example({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text("Splash Screen Example"),
+      ),
+      body: Center(
+        child: Text("Hello World"),
+      ),
+    );
+  }
+}

enabling_splash_screen/lib/main.dart

@@ -1,6 +1,8 @@
 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});
@@ -10,14 +12,7 @@ class MyApp extends StatelessWidget {
   Widget build(BuildContext context) {
     return MaterialApp(
       title: 'Flutter Demo',
-      home: Scaffold(
-        appBar: AppBar(
-          title: Text("Splash Screen Example"),
-        ),
-        body: Center(
-          child: Text("Hello World"),
-        ),
-      ),
+      home: const Example(),
     );
   }
 }