Issue #3611785: Stateful booking cart, seated booking UX and configurable workflow start

See #3611785 for the full summary.

Big feature branch: node-based booking display, per-resource Slots tab and venue Schedule tab, seat map with seated auto-assignment, printed row/place seat labels, and a stateful per-session cart with an order state machine (empty, pending, locked, placed, confirmed, cancelled) and a configurable per-resource workflow start point.