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/Source.cs

@@ -238,7 +238,7 @@ public class Source : SceneElement
         set => Set(ref _alertDisplayMethod, value);
     }
 
-    public bool IsAlertBox => Type == SourceType.AlertBox;
+    public override bool IsAlertBox => Type == SourceType.AlertBox;
 
     // Chat box config (TASK 3.18)
     private double _chatFontSize = 16;