Add member to class: capacity, waiting list, and payment improved

We have improved the coach flow for adding a member to a class.

If a class is full, the coach now clearly sees which choice is available: adding the member anyway if transferring is allowed, or placing the member on the waiting list. If the hard group limit is enabled, adding the member to the class is blocked, while placing them on the waiting list remains possible if available.

The payment step has also been improved: product/credit bookings are checked in advance, so that a coach sees before confirming when no suitable product is available. The invoice price is populated with the lesson price by default when known, and otherwise displays a clear helper text.