feat(notifications): support dismissing notifications in PostIt

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/Yavsc.Abstract/Messaging/DismissedNotification.cs

@@ -0,0 +1,11 @@
+namespace Yavsc.Abstract.Models.Messaging
+{
+    /// <summary>
+    /// Wire format of a notification dismissed by the current user.
+    /// The user is always implied by the authenticated caller.
+    /// </summary>
+    public sealed class DismissedNotification
+    {
+        public long NotificationId { get; set; }
+    }
+}