TASK 9.4: live chat wiring — right panel with Super Chat + membership badges

- YouTubeStreamService: added GetBroadcastLiveChatIdAsync to fetch liveChatId from broadcast contentDetails
- YouTubeChatService.Poll(): parse snippet.type to handle textMessageEvent, superChatEvent, newSponsorEvent
- MainViewModel: wire _youtubeChat.Start(liveChatId) in PrepareAndStartLiveAsync after broadcast creation
- MainViewModel: wire _youtubeChat.Stop() in StopStream before encoder stop
- MainViewModel: wire _youtubeChat.Dispose() in Shutdown
- MainWindow.xaml: updated chat DataTemplate with member badge (⭐), Super Chat highlight (💰 + gold border), timestamps (HH:mm)
- MainWindow.xaml.cs: auto-scroll ChatListBox on new messages via CollectionChanged
- Chat lifecycle is non-fatal: failures log and the stream continues without on-panel chat

MainWindow.xaml.cs

@@ -21,6 +21,7 @@ public partial class MainWindow : Window
         AppLog.Write("MainWindow ctor: after InitializeComponent");
         _viewModel = (MainViewModel)DataContext;
         _viewModel.PropertyChanged += OnViewModelPropertyChanged;
+        _viewModel.ChatMessages.CollectionChanged += OnChatMessagesChanged;
         // When a focused scene is deleted, never land focus on a hidden scene.
         SceneList.IsSelectable = item => item is Scene { IsHidden: false };
         UpdateTaskbarOverlay();
@@ -232,6 +233,14 @@ public partial class MainWindow : Window
         TakeSnapshot();
     }
 
+    private void OnChatMessagesChanged(object? sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
+    {
+        if (e.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Add)
+        {
+            ChatListBox.ScrollIntoView(ChatListBox.Items[^1]);
+        }
+    }
+
     // Static images and the webcam share the move/resize/selection behavior.
     private static bool IsDraggableElement(SceneElement element)
         => element.IsDraggable;