refactor: split app wrapper from focused example code (pilot)

Apply a consistent structure to 5 example apps (simple_material_app,
stateful_widget, dropdown_button, using_gradient, using_tabs):

- lib/main.dart now contains only the app bootstrap: runApp + a const
  MyApp widget wrapping the app in a MaterialApp.
- lib/example.dart holds the full screen (Scaffold, AppBar, and the
  focused code that demonstrates the example feature), marked with a
  '// Example:' banner comment.

This makes it easy to pinpoint the code that teaches each concept while
keeping the wrapping/chrome code uniform and minimal. Review this pilot
before rolling the pattern out to the remaining apps.

using_gradient/lib/example.dart

@@ -0,0 +1,33 @@
+import 'package:flutter/material.dart';
+
+import 'utils.dart' as utils;
+
+// Example: the focused code for this app.
+// A Container whose background uses a custom LinearGradient.
+class Example extends StatelessWidget {
+  const Example({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text("Using Gradient"),
+      ),
+      body: Container(
+        // Decoration
+        decoration: BoxDecoration(
+          // Add Gradient
+          gradient: utils.getCustomGradient(),
+        ),
+        // Center the content
+        child: const Center(
+          // Add Text
+          child: Text(
+            "Hello World!",
+            style: TextStyle(color: Colors.white),
+          ),
+        ),
+      ),
+    );
+  }
+}

using_gradient/lib/main.dart

@@ -1,30 +1,19 @@
 import 'package:flutter/material.dart';
 
-import './utils.dart' as utils;
+import 'example.dart';
 
 void main() {
-  runApp(MaterialApp(
-      // Title
-      title: "Using Gradient",
-      // Home
-      home: Scaffold(
-          // Appbar
-          appBar: AppBar(
-            // Title
-            title: Text("Using Gradient"),
-          ),
-          // Body
-          body: Container(
-              // Center the content
-              decoration: BoxDecoration(
-                  // Add Gradient
-                  gradient: utils.getCustomGradient()),
-              // Center the content
-              child: Center(
-                // Add Text
-                child: Text(
-                  "Hello World!",
-                  style: TextStyle(color: Colors.white),
-                ),
-              )))));
+  runApp(const MyApp());
+}
+
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: "Using Gradient",
+      home: const Example(),
+    );
+  }
 }