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_snackbar/lib/example.dart

@@ -0,0 +1,49 @@
+import 'package:flutter/material.dart';
+
+// Example: show a SnackBar with an action button when the button is pressed.
+class Example extends StatelessWidget {
+  const Example({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text("Using SnackBar"),
+      ),
+      body: Center(
+        child: MyButton(),
+      ),
+    );
+  }
+}
+
+class MyButton extends StatelessWidget {
+  const MyButton({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return ElevatedButton(
+      child: const Text('Show SnackBar'),
+      // On pressing the raised button
+      onPressed: () {
+        // show snackbar
+        ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+          // set content of snackbar
+          content: const Text("Hello! I am SnackBar :)"),
+          // set duration
+          duration: const Duration(seconds: 3),
+          // set the action
+          action: SnackBarAction(
+              label: "Hit Me (Action)",
+              onPressed: () {
+                // When action button is pressed, show another snackbar
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+                  content: const Text(
+                      "Hello! I am shown becoz you pressed Action :)"),
+                ));
+              }),
+        ));
+      },
+    );
+  }
+}

using_snackbar/lib/main.dart

@@ -1,52 +1,20 @@
 import 'package:flutter/material.dart';
 
+import 'example.dart';
+
 void main() {
-  runApp(MaterialApp(home: ContactPage(), debugShowCheckedModeBanner: false,));
+  runApp(const MyApp());
 }
 
-class ContactPage extends StatelessWidget {
-  const ContactPage({super.key});
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
 
   @override
   Widget build(BuildContext context) {
-    return Scaffold(
-      appBar: AppBar(
-        title: Text("Using SnackBar"),
-      ),
-      body: Center(
-        child: MyButton(),
-      ),
-    );
-  }
-}
-
-class MyButton extends StatelessWidget {
-  const MyButton({super.key});
-
-  @override
-  Widget build(BuildContext context) {
-    return ElevatedButton(
-      child: Text('Show SnackBar'),
-      // On pressing the raised button
-      onPressed: () {
-        // show snackbar
-        ScaffoldMessenger.of(context).showSnackBar(SnackBar(
-              // set content of snackbar
-              content: Text("Hello! I am SnackBar :)"),
-              // set duration
-              duration: Duration(seconds: 3),
-              // set the action
-              action: SnackBarAction(
-                  label: "Hit Me (Action)",
-                  onPressed: () {
-                    // When action button is pressed, show another snackbar
-                    ScaffoldMessenger.of(context).showSnackBar(SnackBar(
-                          content: Text(
-                              "Hello! I am shown becoz you pressed Action :)"),
-                        ));
-                  }),
-            ));
-      },
+    return MaterialApp(
+      debugShowCheckedModeBanner: false,
+      title: "Using SnackBar",
+      home: const Example(),
     );
   }
 }