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/example.dart

@@ -0,0 +1,21 @@
+import 'package:flutter/material.dart';
+
+// Example: the focused code for this app.
+// Shows a single centered Text widget inside a plain Material app.
+class Example extends StatelessWidget {
+  const Example({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text("Simple Material App"),
+      ),
+      body: Container(
+        child: const Center(
+          child: Text("Hello World!"),
+        ),
+      ),
+    );
+  }
+}