Issue #3611904: Keep the seat map's total and Remove all button visible when the selection list is long

Follow-up to #3611898 (the Remove all button).

Problem

The "Your selection" panel is height-capped beside the map. With enough seats selected the list scrolls, and the total and the Remove all button scrolled out of view with it, so they could not be reached.

Change

Wrap the total and the Remove-all button in a .ysm-foot footer with position: sticky; bottom: 0 (over the panel background). They sit just below the list when it is short, and stick to the bottom of the panel once the list is long enough to scroll. The panel's bottom padding moves into the footer so it reaches the panel's edge and the scrolling picks do not show through beneath it. CSS + one scaffold line only; no new strings.