- settings.py: add orders.waiter_cancellations_allowed to VALID_SETTINGS
(was missing, causing 400 on every PUT attempt)
- StaffTab.jsx: include can_cancel_orders in edit waiter mutation payload
(was omitted, so the checkbox change was never sent to the backend)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>