fix: chatbox properties panel showing when webcam is selected

SceneElement was missing virtual IsChatBox property (unlike IsWebcam, IsWebSource, IsImageSource).
WPF binding failed silently when SelectedElement was a webcam, defaulting StackPanel to Visible.
Added virtual bool IsChatBox => false on SceneElement, made Source.IsChatBox override it.

Models/Source.cs

@@ -182,7 +182,7 @@ public class Source : SceneElement
         set => Set(ref _chatFadeSeconds, Math.Clamp(value, 0, 60));
     }
 
-    public bool IsChatBox => Type == SourceType.ChatBox;
+    public override bool IsChatBox => Type == SourceType.ChatBox;
 
     private bool _chatPreviewEnabled = true;
     public bool ChatPreviewEnabled