fix: countdown timer panel showing under webcam properties

Same root cause as IsChatBox — SourceType binding fails on WebcamSceneConfig,
WPF defaults Visibility to Visible without calling the converter.
Added virtual bool IsCountdown on SceneElement, override in Source.
Swapped binding to BoolToVis. Removed dead SourceTypeToVisibility converters.

Models/SceneElement.cs

@@ -59,6 +59,9 @@ public abstract class SceneElement : INotifyPropertyChanged
     /// <summary>True only for ChatBox-type sources. Gates chat overlay properties panel.</summary>
     public virtual bool IsChatBox => false;
 
+    /// <summary>True only for Countdown-type sources. Gates timer properties panel.</summary>
+    public virtual bool IsCountdown => false;
+
     private bool _isVisible = true;
     public bool IsVisible { get => _isVisible; set => Set(ref _isVisible, value); }