* Add customizable notification sound setting
Adds a "Notification Sound" picker in Settings > App that lets users
choose from macOS system sounds (Default, Basso, Blow, Glass, etc.)
or silence notifications entirely with "None".
Closes https://github.com/manaflow-ai/cmux/issues/608
* Add custom notification command with env vars and sound preview
Users can set a shell command in Settings > App > Notification Command
that runs on every notification. CMUX_NOTIFICATION_TITLE,
CMUX_NOTIFICATION_SUBTITLE, and CMUX_NOTIFICATION_BODY env vars are
set. Also adds a play button to preview system sounds and docs.
a5de92e9d6
