AdminRequestsScreen renders rows from adminRequestsProvider, joining
requester user_id → username via adminUsersProvider for display
(falls back to UUID prefix when the users list hasn't loaded yet).
Approve fires immediately; Reject confirms via dialog. Both are
optimistic with rollback in the controller.
Adds AdminUsersController build-only (the read side) so the row join
works; mutation methods land in Slice 3 alongside the Users screen.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
