YouTube Chat layer: one-per-layout gate + legacy label heal (TASK 27)

ViewModels/MainViewModel.cs

@@ -213,6 +213,7 @@ public class MainViewModel : ViewModelBase
                 OnPropertyChanged(nameof(ShowSourcesEmptyHint));
                 OnPropertyChanged(nameof(CanChangeBackground));
                 OnPropertyChanged(nameof(CanAddWebcam));
+                OnPropertyChanged(nameof(CanAddYouTubeChat));
                 OnPropertyChanged(nameof(CanShowWebcamInStagedScene));
                 OnPropertyChanged(nameof(IsCountdownAvailable));
                 UpdateActiveBackground();
@@ -307,6 +308,12 @@ public class MainViewModel : ViewModelBase
     /// one camera identity app-wide; re-adding always opens the picker.</summary>
     public bool CanAddWebcam => StagedScene != null && _webcam == null;
 
+    /// <summary>The Add → YouTube Chat menu item: enabled only while no chat layer exists
+    /// anywhere — one chat overlay per layout.</summary>
+    public bool CanAddYouTubeChat
+        => StagedScene != null
+           && !Scenes.Any(s => s.Elements.OfType<Source>().Any(e => e.Type == SourceType.ChatBox));
+
     // ─── Transitions (TASK 19/23) ───
 
     public TransitionType SelectedTransitionType
@@ -1770,6 +1777,15 @@ public class MainViewModel : ViewModelBase
                     ClampWebcamToBounds(config, scene.Name);
                     HealLegacySquareRect(config);
                 }
+
+            // One-time heal: ChatBox layers created before the "YouTube Chat"
+            // default still carry the old "Chat" name — creator renames untouched.
+            foreach (var scene in Scenes)
+                foreach (var source in scene.Elements.OfType<Source>())
+                    if (source.Type == SourceType.ChatBox
+                        && string.Equals(source.Name, "Chat", StringComparison.OrdinalIgnoreCase))
+                        source.Name = "YouTube Chat";
+
             AppLog.Write($"LoadLayout: {Scenes.Count} scenes loaded");
         }
         finally
@@ -2264,6 +2280,7 @@ public class MainViewModel : ViewModelBase
             foreach (SceneElement element in e.OldItems)
                 element.PropertyChanged -= OnElementPropertyChanged;
         OnPropertyChanged(nameof(CanAddWebcam));
+        OnPropertyChanged(nameof(CanAddYouTubeChat));
         ScheduleSave();
     }
 
@@ -2383,9 +2400,11 @@ public class MainViewModel : ViewModelBase
 
         // The (+) menu only adds regular layers. The Background is a locked
         // per-screen layer managed by EnsureBackground/NormalizeBackgrounds —
-        // never user-created, never deleted.
+        // never user-created, never deleted. YouTube Chat is one-per-layout.
         if (sourceType is SourceType.DisplayCapture or SourceType.WindowCapture or SourceType.Background)
             return;
+        if (sourceType == SourceType.ChatBox && !CanAddYouTubeChat)
+            return;
 
         var source = new Source
         {