[Tbaile]

Added the superuser concept: other admins can no longer change SSH settings, manage administrators, handle backups and a few more system-level actions

• Replaced the old SSH management with a proper ns.ssh API
• Added ns.account get-info so the UI can hide or disable what the logged user can't use
• Better cleanup when deleting or demoting a user, their sessions and 2FA secret are removed too

Closes: #1721