feat(alerts): add scene-local recent ticker

Model the AlertBox and standalone ticker with shared settings; scene tickers render the in-memory active and queued events without saving history. Keep AlertBox content active-event-only.

Ref: https://obsproject.com/kb/obs-studio-overview

ViewModels/MainViewModel.Chat.cs

@@ -44,6 +44,11 @@ public partial class MainViewModel : ViewModelBase
         => StagedScene != null
            && !Scenes.Any(s => s.Elements.OfType<Source>().Any(e => e.Type == SourceType.AlertBox));
 
+    /// <summary>A recent-alert ticker is an independent layer, one per scene.</summary>
+    public bool CanAddTicker
+        => StagedScene != null
+           && !StagedScene.Elements.OfType<Source>().Any(e => e.Type == SourceType.Ticker);
+
     /// <summary>Privacy blur is a Live-scene tool only (creator ruling 2026-11-01) — every other
     /// scene's art is authored, and there is no desktop there to hide.</summary>
     public bool CanAddBlur