Made the Greeting class sealed as it's not suppose to be inherited from

Htmx.ApiDemo/Templates/Greeting.htmx.cs

@@ -1,6 +1,6 @@
 namespace Htmx.ApiDemo.Templates;
 
-public class Greeting : GreetingBase
+public sealed class Greeting : GreetingBase
 {
     private byte[] _userData = [];
     public required string Username { init => _userData = value.ToUtf8Bytes(); }