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.

simple_material_app/lib/main.dart

@@ -1,23 +1,19 @@
 import 'package:flutter/material.dart';
 
+import 'example.dart';
+
 void main() {
-  runApp(MaterialApp(
-      // Title
+  runApp(const MyApp());
+}
+
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
       title: "Simple Material App",
-      // Home
-      home: Scaffold(
-        // Appbar
-        appBar: AppBar(
-          // Title
-          title: Text("Simple Material App"),
-        ),
-        // Body
-        body: Container(
-          // Center the content
-          child: Center(
-            // Add Text
-            child: Text("Hello World!"),
-          ),
-        ),
-      )));
-}
+      home: const Example(),
+    );
+  }
+}