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

ViewModels/MainViewModel.cs

@@ -1605,6 +1605,7 @@ public class MainViewModel : ViewModelBase
         _screenCaptureManager.Dispose();
         _layoutStore.Dispose();
         _musicPlayer.Dispose();
+        _youtubeChat.Dispose();
     }
 
     public void SaveLayoutNow()
@@ -2236,6 +2237,27 @@ public class MainViewModel : ViewModelBase
                 return;
             }
             AppLog.Write($"Broadcast created: {_currentBroadcastId}");
+
+            // Start chat polling (TASK 9.4): fetch the liveChatId from the broadcast's
+            // contentDetails, then start the poll loop. Chat is non-fatal — if it fails,
+            // the stream continues without on-panel chat.
+            try
+            {
+                var liveChatId = await _youtubeStream.GetBroadcastLiveChatIdAsync(_currentBroadcastId);
+                if (liveChatId != null)
+                {
+                    _youtubeChat.Start(liveChatId);
+                    AppLog.Write($"Chat polling started: {liveChatId}");
+                }
+                else
+                {
+                    AppLog.Write("Could not retrieve liveChatId; chat polling disabled");
+                }
+            }
+            catch (Exception chatEx)
+            {
+                AppLog.Write($"Chat start failed (non-fatal): {chatEx.Message}");
+            }
         }
         catch (Exception ex)
         {
@@ -2302,6 +2324,9 @@ public class MainViewModel : ViewModelBase
         StreamStatus = StreamStatus.Offline;
         WindowTitle = "ytLlive";
         ResetHealth(StreamStatus.Offline);
+        // Stop chat polling first (TASK 9.4) — no more messages arriving while
+        // we're tearing down the stream.
+        _youtubeChat.Stop();
         // Close the audio pipe FIRST so ffmpeg sees EOF on the audio input,
         // then stop the pump (stdin EOF) — both inputs end and the encoder
         // finalizes the FLV. Audio capture itself is always-on (preview