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.Sources.cs

@@ -38,6 +38,7 @@ public partial class MainViewModel
             SourceType.WebSource => "Web Resource",
             SourceType.ChatBox => "YouTube Chat",
             SourceType.AlertBox => "Stream Alerts",
+            SourceType.Ticker => "Ticker",
             SourceType.Scoreboard => "Scoreboard",
             _ => "Layer",
         };
@@ -45,11 +46,12 @@ public partial class MainViewModel
         // The (+) menu only adds regular layers. The Background is a locked
         // per-screen layer managed by EnsureBackground/NormalizeBackgrounds —
         // never user-created, never deleted. YouTube Chat and Stream Alerts
-        // are one-per-layout.
+        // are one-per-layout; a ticker is one-per-scene.
         if (sourceType is SourceType.DisplayCapture or SourceType.WindowCapture or SourceType.Background)
             return;
         if ((sourceType == SourceType.ChatBox && !CanAddYouTubeChat)
-            || (sourceType == SourceType.AlertBox && !CanAddAlerts))
+            || (sourceType == SourceType.AlertBox && !CanAddAlerts)
+            || (sourceType == SourceType.Ticker && !CanAddTicker))
             return;
 
         // Web resources use "Web Resource-N" naming (0-indexed per scene)
@@ -73,11 +75,13 @@ public partial class MainViewModel
                 : sourceType == SourceType.ChatBox ? 500
                 : sourceType == SourceType.AlertBox ? 680
                 : sourceType == SourceType.Scoreboard ? 1920
+                : sourceType == SourceType.Ticker ? 1920
                 : sourceType == SourceType.Blur ? 240 : 0,
             Height = sourceType == SourceType.WebSource ? 480
                 : sourceType == SourceType.ChatBox ? 648
                 : sourceType == SourceType.AlertBox ? 200
                 : sourceType == SourceType.Scoreboard ? 56
+                : sourceType == SourceType.Ticker ? 48
                 : sourceType == SourceType.Blur ? 160 : 0,
         };