Add check to both user and admin cancel endpoints to reject
cancellation of appointments whose slot_start is in the past.
This matches the spec requirement that cancellations can only
happen 'before the appointment'.
Added tests for both user and admin cancel endpoints.
Also includes frontend styling updates.
- Use data-testid attributes to target specific day cards in availability tests
- Wait for networkidle before interacting with page elements
- Set up PUT and GET response listeners before triggering actions
- Add retry logic for availability API in booking tests
- Fix appointments test to handle multiple 'Booked' elements with .first()
- Increase parallel workers to 12 for faster test execution
