Admin UI user cart and session management

[Mikearaya]

Add user cart tab and logout-all-sessions support to admin-ui

• Show the active cart of a user on the user detail page via the
  user.cart field, with items, totals, and an admin-only link to
  the order detail view
• Add a Sessions section to the account view: "Log out everywhere"
  for the own account, "Force logout" for admins on other users
• Extend the logoutAllSessions mutation with an optional userId so
  admins can invalidate another user's sessions; gated by the
  existing isMyself ACL check and covered by integration tests