feat: implement channel membership management and ensure default channels are public

src/EchoHub.Client/UI/MainWindow.cs

@@ -596,6 +596,16 @@ public sealed class MainWindow : Runnable
         RefreshChannelList();
     }
 
+    /// <summary>
+    /// Remove a channel from the left panel list.
+    /// </summary>
+    public void RemoveChannel(string channelName)
+    {
+        _channelNames.Remove(channelName);
+        _channelTopics.Remove(channelName);
+        RefreshChannelList();
+    }
+
     /// <summary>
     /// Update the topic for a specific channel.
     /// </summary>