Break code into multiple files.

lib/pages/settings.dart

@@ -0,0 +1,12 @@
+import 'package:flutter/material.dart';
+
+class SettingsPage extends StatelessWidget {
+  const SettingsPage({
+    super.key,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return Placeholder();
+  }
+}