Fix: add IsAlertBox virtual on SceneElement and override on Source

The Stream Alerts properties panel was incorrectly showing for the webcam
layer (a WebcamSceneConfig is not a Source) because IsAlertBox only existed
on Source, causing WPF binding to be treated as false-but-invisible context
or the trigger to bind against the wrong type. Adding a virtual IsAlertBox
to the base SceneElement (default false) and overriding on Source prevents
the UI block from rendering under webcam selection. Scope: Models/SceneElement.cs, Models/Source.cs, ytLive.Tests/AlertBoxPreviewVisibilityTests.cs. Build/test: app + 4 alert-box tests pass, other failures unrelated (float precision).

Models/SceneElement.cs

@@ -77,6 +77,9 @@ public abstract class SceneElement : INotifyPropertyChanged
 
     /// <summary>True only for Countdown-type sources. Gates timer properties panel.</summary>
     public virtual bool IsCountdown => false;
+
+    /// <summary>True only for AlertBox-type sources. Gates stream alerts properties panel.</summary>
+    public virtual bool IsAlertBox => false;
     public virtual bool IsBackground { get; set; }
 
     private bool _useDefaultBackground = true;