Choose the telnet window's colours on the window thread

The window picked a brush on the socket thread, and Avalonia crashed the
program the next time it painted that line. A line that arrives is now
posted to the window thread first. Spots no longer show the bell
character as a box, and an unexpected error in the cluster loop is
shown instead of ending it silently.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/Nonemm.Spotting/ClusterClient.cs

@@ -170,6 +170,12 @@ public sealed class ClusterClient : IDisposable
         catch (OperationCanceledException)
         {
         }
+        catch (Exception e)
+        {
+            // anything else would end the loop with nothing on the screen
+            Notice($"*** {Host}:{Port} stopped: {e}");
+            SetConnected(false);
+        }
     }
 
     private async Task SessionAsync(CancellationToken cancellation)