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/Source.cs

@@ -38,12 +38,15 @@ public class Source : SceneElement
             {
                 Raise(nameof(IsLiveCapture));
                 Raise(nameof(DisplaySource));
+                Raise(nameof(IsWebSource));
             }
         }
     }
 
     public override bool IsDraggable => Type == SourceType.Image;
 
+    public override bool IsWebSource => Type == SourceType.WebSource;
+
     /// <summary>The permanent scene backdrop (live desktop/game capture). Never removable/reorderable.</summary>
     private bool _isBackdrop;
     public bool IsBackdrop { get => _isBackdrop; set => Set(ref _isBackdrop, value); }