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.App/Windows/TelnetWindow.axaml.cs

@@ -107,8 +107,15 @@ public sealed partial class TelnetWindow : RefreshableWindow
         cluster.ConnectionChanged -= OnConnectionChanged;
     }
 
-    private void OnLineArrived(object? sender, string line)
+    /// The cluster raises its events on a socket thread, and a brush belongs to
+    /// the thread that made it, so the colour is chosen on the window thread.
+    private void OnLineArrived(object? sender, string line) =>
+        Dispatcher.UIThread.Post(() => ShowArrived(line));
+
+    private void ShowArrived(string line)
     {
+        // nodes end a spot with the bell character, which draws as a box
+        line = line.Replace("\a", "");
         if (line.StartsWith("***", StringComparison.Ordinal))
         {
             Show(line, NoticeColour);
@@ -128,7 +135,8 @@ public sealed partial class TelnetWindow : RefreshableWindow
         session.Settings.SpotFilter.PreferredSpotters.Any(one =>
             one.Length > 0 && spotter.StartsWith(one, StringComparison.OrdinalIgnoreCase));
 
-    private void OnLineSent(object? sender, string line) => Show($"> {line}", SentColour);
+    private void OnLineSent(object? sender, string line) =>
+        Dispatcher.UIThread.Post(() => Show($"> {line}", SentColour));
 
     private void OnConnectionChanged(object? sender, bool connected) =>
         Dispatcher.UIThread.Post(Refresh);

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)