Share workspace operations between owners and admins

Workspaces can now share day-to-day operations among multiple admins while keeping one owner responsible for billing and role assignments.

Owners can appoint admins and transfer ownership to another member. Admins can remove and restore members, transfer artifacts, and manage the workspace name, Slack integration, Team projects, and comments.

Checkout, plan and billing management, the customer portal, admin appointments, and ownership transfer remain owner-only. The Settings page now shows each role and links to public guides for owners and admins.