Reshape the slot booking page: a flat column of quantity spinners with price as helper text

The slot booking page is one number field per slot tier, stacked, and little else. SlotBookingForm::buildForm() loops the slot's tiers and emits quantityField() for each; that field is a number element whose #title is the price category's label and whose #description is the price and the word Available. Everything a buyer decides on is therefore helper text under a spinner.

Concrete problems, in rough order of how much they hurt:

• The tariff matrix is flattened into labels. Sites price a seating grade by a tariff family, so the tiers are a cross-product ("Full price - Category 1", "Youth - Category 1", ...). The page renders them as N sibling spinners and the grouping survives only as a string inside each label. A hall with four grades and three tariff families is twelve stacked spinners with no rows, no columns and no headings.
• Price has the lowest visual weight on the page. It is part of #description, joined to the availability word with a dash. There is no price column, no per-line total, and the only total is a bare paragraph at the bottom.
• Availability is a single word. Deliberately so, because the raw count is racy under concurrent booking, but the result is that scarcity is invisible: nothing distinguishes a hall with two seats left from a hall with two thousand. A banded hint ("a few left") would keep the honesty and restore the signal.
• The maximum is enforced silently. The field's #max is the held quantity plus what is free, so the spinner simply refuses to go higher with no explanation of the ceiling.
• The order block re-implements the summary, worse. It is an item_list of bare place labels, so a line reads "A15" with no tariff, no area and no price. The module already has a richer renderer: TransactionSummary produces tier, seat, area and price per line through a theme hook, and the placement module already contributes the area name to it. Worse, a printed seat name is only unique within its area, so on a real venue several different seats are all printed "A15" and the line does not identify a seat at all.
• Nothing connects a tariff to where the seats are. The page never says which area a grade corresponds to, and the map is a separate page, so a buyer choosing between two prices is choosing between two numbers.
• Two submit buttons, no explanation. Update basket and Continue to checkout sit side by side, and the order block only reflects a change after the first one is pressed.

What a reshape should probably do: group the tiers by their tariff family and their grade rather than emitting siblings, give price and availability real positions instead of helper text, render the order through TransactionSummary so the lines say what they are, and show enough area context that a price means something. Whether the quantity control stays a spinner is an open question; for a small party a stepper or a select reads better and avoids the silent cap.

Scope note: fixing only the order block (routing it through TransactionSummary, which also restores the per-line prices and the area) is a small, self-contained improvement and could land first if the wider reshape needs design discussion.