Fix: URI text box no longer shows under webcam source

The web source section's visibility was bound to SelectedElement.Type
via a converter, but WebcamSceneConfig has no Type property — the WPF
binding path didn't resolve correctly and the section leaked through.

Added IsWebSource virtual property to SceneElement (mirrors existing
IsWebcam pattern), overridden in Source to return
Type == SourceType.WebSource. Section visibility now binds directly to
SelectedElement.IsWebSource via BoolToVis.

1c of 7-issue polish batch. 207 tests, 0 warnings.

Models/SceneElement.cs

@@ -47,6 +47,9 @@ public abstract class SceneElement : INotifyPropertyChanged
     /// <summary>True only for singleton resources (webcam). Gates webcam-only UI.</summary>
     public virtual bool IsWebcam => false;
 
+    /// <summary>True only for WebSource-type elements. Gates URI input visibility.</summary>
+    public virtual bool IsWebSource => false;
+
     /// <summary>True for elements that can be dragged/resized in the preview (image, webcam).</summary>
     public virtual bool IsDraggable => false;