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/Source.cs

@@ -184,6 +184,8 @@ public class Source : SceneElement
 
     public override bool IsChatBox => Type == SourceType.ChatBox;
 
+    public override bool IsCountdown => Type == SourceType.Countdown;
+
     private bool _chatPreviewEnabled = true;
     public bool ChatPreviewEnabled
     {