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.AlertTicker.cs

@@ -1,3 +1,5 @@
+using System.Linq;
+using ytLive.Models;
 using ytLive.Services;
 
 namespace ytLive.ViewModels;
@@ -44,8 +46,15 @@ public partial class MainViewModel
     /// Reuses one <see cref="System.Windows.Media.Imaging.WriteableBitmap"/> and
     /// overwrites its back buffer — a fresh bitmap every tick would churn the
     /// render cache for no reason.</summary>
-    internal void PublishAlertTickerPreview(VideoFrame? frame)
+    internal void PublishAlertTickerPreview(VideoFrame? incomingFrame)
     {
+        var frame = incomingFrame;
+        var previewScene = IsEditMode ? StagedScene : LiveScene ?? StagedScene;
+        var alertBox = previewScene?.Elements.OfType<Source>()
+            .FirstOrDefault(source => source.Type == SourceType.AlertBox && source.IsVisible);
+        if (_alertLayer.IsPlaying)
+            frame = _alertLayer.AlertBoxTickerFrame(alertBox);
+
         if (frame == null)
         {
             if (_alertTickerBitmap != null)