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

Services/YouTubeChatService.cs

@@ -73,6 +73,7 @@ public class YouTubeChatService : IDisposable
             {
                 var snippet = item.GetProperty("snippet");
                 var author = item.GetProperty("authorDetails");
+                var type = snippet.GetProperty("type").GetString() ?? "textMessageEvent";
 
                 var message = new ChatMessage
                 {
@@ -80,11 +81,50 @@ public class YouTubeChatService : IDisposable
                     AuthorName = author.GetProperty("displayName").GetString()!,
                     AuthorChannelId = author.GetProperty("channelId").GetString()!,
                     AuthorImageUrl = author.GetProperty("profileImageUrl").GetString()!,
-                    Message = snippet.GetProperty("textMessageDetails").GetProperty("messageText").GetString()!,
                     Timestamp = DateTime.Parse(snippet.GetProperty("publishedAt").GetString()!),
-                    IsMember = author.GetProperty("isChatSponsor").GetBoolean()
+                    IsMember = author.GetProperty("isChatSponsor").GetBoolean(),
+                    MembershipLevel = author.TryGetProperty("badgeText", out var badgeText)
+                        ? badgeText.GetString()
+                        : null
                 };
 
+                switch (type)
+                {
+                    case "superChatEvent":
+                        message.IsSuperChat = true;
+                        if (snippet.TryGetProperty("superChatDetails", out var scDetails))
+                        {
+                            message.SuperChatAmount = scDetails.TryGetProperty("amountMicros", out var amt)
+                                ? amt.GetDouble() / 1_000_000.0
+                                : 0;
+                            message.SuperChatCurrency = scDetails.TryGetProperty("currency", out var cur)
+                                ? cur.GetString()
+                                : null;
+                            message.Message = scDetails.TryGetProperty("userComment", out var comment)
+                                ? comment.GetString() ?? string.Empty
+                                : string.Empty;
+                        }
+                        break;
+
+                    case "newSponsorEvent":
+                        message.Message = $"🎉 {message.AuthorName} became a member!";
+                        if (snippet.TryGetProperty("membershipDetails", out var memDetails))
+                        {
+                            message.MembershipLevel = memDetails.TryGetProperty("memberLevelName", out var level)
+                                ? level.GetString()
+                                : message.MembershipLevel;
+                        }
+                        break;
+
+                    default: // textMessageEvent
+                        message.Message = snippet.TryGetProperty("textMessageDetails", out var textDetails)
+                            ? textDetails.TryGetProperty("messageText", out var text)
+                                ? text.GetString() ?? string.Empty
+                                : string.Empty
+                            : string.Empty;
+                        break;
+                }
+
                 MessageReceived?.Invoke(message);
             }
         }

Services/YouTubeStreamService.cs

@@ -190,4 +190,28 @@ public class YouTubeStreamService
         }
         return health;
     }
+
+    /// <summary>Returns the liveChatId for a broadcast by fetching its contentDetails.
+    /// The liveChatId is needed to poll chat messages. Called after broadcast creation
+    /// in PrepareAndStartLiveAsync.</summary>
+    public async Task<string?> GetBroadcastLiveChatIdAsync(string broadcastId)
+    {
+        if (!await EnsureToken()) return null;
+
+        _http.DefaultRequestHeaders.Authorization = new("Bearer", _auth.CurrentChannel!.AccessToken);
+        var response = await _http.GetAsync(
+            $"{ApiBase}/liveBroadcasts?part=contentDetails&id={broadcastId}");
+        if (!response.IsSuccessStatusCode) return null;
+
+        var json = await response.Content.ReadAsStringAsync();
+        var data = JsonSerializer.Deserialize<JsonElement>(json);
+        var items = data.GetProperty("items");
+        if (items.GetArrayLength() == 0) return null;
+
+        var contentDetails = items[0].GetProperty("contentDetails");
+        if (contentDetails.TryGetProperty("liveChatId", out var liveChatId))
+            return liveChatId.GetString();
+
+        return null;
+    }
 }