declare StartupWMClass in .desktop (#4930)

This allows the app to be pinned in the dock/task manager in several
common desktop environments.

When not set this happens when you open the pinned application: 


![image](https://github.com/user-attachments/assets/81316c9c-4cd4-4a7c-ba3e-74b9a7abc1fa)

dist/linux/app.desktop

@@ -7,6 +7,7 @@ Icon=com.mitchellh.ghostty
 Categories=System;TerminalEmulator;
 Keywords=terminal;tty;pty;
 StartupNotify=true
+StartupWMClass=com.mitchellh.ghostty
 Terminal=false
 Actions=new-window;
 X-GNOME-UsesNotifications=true