nav [nfc]: Use a buildRoute pattern for MessageListPage

This lets the widget class better encapsulate what its route
is meant to look like.  That will help us as we start to do
slightly more complex things with the routes.

Author: gnprice

lib/widgets/app.dart

@@ -71,8 +71,7 @@ class HomePage extends StatelessWidget {
           const SizedBox(height: 16),
           ElevatedButton(
             onPressed: () => Navigator.push(context,
-              MaterialAccountPageRoute(context: context, builder: (context) =>
-                const MessageListPage())),
+              MessageListPage.buildRoute(context)),
             child: const Text("All messages")),
         ])));
   }
@@ -81,6 +80,11 @@ class HomePage extends StatelessWidget {
 class MessageListPage extends StatelessWidget {
   const MessageListPage({super.key});
 
+  static Route<void> buildRoute(BuildContext context) {
+    return MaterialAccountPageRoute(context: context,
+      builder: (context) => const MessageListPage());
+  }
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(