* feat(auth): add per-user admin promote/demote toggle
Admin-only API and Users-tab control to grant/revoke admin rights; refuses to demote the last admin.
* fix(auth): restore pre-admin privilege restrictions on demotion
Promoting now stashes the user's privilege map (privileges_before_admin)
and demoting restores it instead of resetting to defaults, so a
promote/demote round trip can no longer broaden a restricted user's
access. Users without a stash (created as admin, or promoted before this
fix) still demote to DEFAULT_PRIVILEGES so a born-admin's stored all-True
map — including can_use_bash — can't survive demotion.
---------
Co-authored-by: K M Merajul Arefin <merajul.arefin@therapservices.net>
Merajul Arefin