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.cs

@@ -405,7 +405,8 @@ public partial class MainViewModel : ViewModelBase
             encoderFactory: () => new FfmpegEncoder(new FfmpegLocator()),
             log: message => AppLog.Write(message),
             socialBar: () => (_socialBarFrame, _socials?.BarPosition ?? SocialBarPosition.Bottom),
-            alertTicker: () => _alertLayer.AlertTickerFrame,
+            alertTicker: scene => _alertLayer.AlertBoxTickerFrame(
+                scene.Elements.OfType<Source>().FirstOrDefault(source => source.Type == SourceType.AlertBox && source.IsVisible)),
             brandFlash: BrandFlashFrame,
             transition: _transition,
             sceneGraph: _sceneGraph);
@@ -639,6 +640,7 @@ public partial class MainViewModel : ViewModelBase
             WebcamSceneConfig webcam => _cameraManager.GetLatestFrame(DeviceKeyForWebcam(webcam.WebcamId), EffectKeyFor(webcam)),
             Source { IsLiveCapture: true, CaptureKey: not null } live => _screenCaptureManager.GetLatestFrame(live.CaptureKey),
             Source { Type: SourceType.ChatBox } chatBox => RenderChatBox(chatBox),
+            Source { Type: SourceType.Ticker } ticker => _alertLayer.RenderSceneTicker(ticker),
             Source { Type: SourceType.Scoreboard } => _scoreboardPresenter?.Frame,
             Source { Type: SourceType.AlertBox } alertBox => RenderAlertBox(alertBox),
             Source { Type: SourceType.WebSource } web => _webView2Manager?.GetLatestFrame(web.Id),