fix(windows): polish hotkeys, user list, titlebar, and PM window

- Suppress the system ding on global hotkeys/PTT and the user-list Enter
  key by setting SuppressKeyPress (Handled alone leaves WM_CHAR to beep).
- Preserve the user-list keyboard selection across talking/mute refreshes
  instead of resetting it on every Items.Clear().
- Include the connected server name in the main window titlebar.
- Close the private-message window on Escape.
- Show the PM window without an owner so focus is no longer trapped to it
  and the main window can be worked in while a PM is open.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clients/windows/VoiceCat.App/Forms/PrivateMessageForm.cs

@@ -26,6 +26,11 @@ public sealed class PrivateMessageForm : Form
         ClientSize = new Size(480, 360);
         MinimumSize = new Size(320, 240);
         StartPosition = FormStartPosition.Manual;
+        KeyPreview = true;
+        KeyDown += (_, e) =>
+        {
+            if (e.KeyCode == Keys.Escape) { e.Handled = e.SuppressKeyPress = true; Close(); }
+        };
 
         _rtbHistory = new RichTextBox
         {