Clip cards that start on first booking can book earlier classes again

A pass that starts on its first booking anchors its validity window to the date of the first class booked. If that first class was some days out, booking any EARLIER class was refused with "no valid pass" — clients experienced this as their clip card suddenly limiting how much they could book.

Booking an earlier class now automatically moves the pass window to that class (as it already did when a specific pass was chosen explicitly), as long as every existing booking still fits the moved window. If a later booking would fall outside, the client gets a clear choice instead of a silent failure.