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/SceneElement.cs

@@ -56,6 +56,9 @@ public abstract class SceneElement : INotifyPropertyChanged
     /// <summary>True only for image sources — the raw preview renderer's discriminator.</summary>
     public virtual bool IsImageSource => false;
 
+    /// <summary>True only for ChatBox-type sources. Gates chat overlay properties panel.</summary>
+    public virtual bool IsChatBox => false;
+
     private bool _isVisible = true;
     public bool IsVisible { get => _isVisible; set => Set(ref _isVisible, value); }