Issue #3614909: Offer cards print their price in different places when one of them has a selling limit

The limit was the quantity field's description, so a theme rendered it inside the field's own column; the sentence is wider than the stepper, the column grew, and the price of that one card moved left of every other card's price.

The card carries the sentence itself now, on its own line under the price and the control, which become one group ending where every other card's group ends. The field points at the line with aria-describedby, which core wired for it while it was a description, so a keyboard user still hears why the plus button stops.

Two cascade ties lost to later stylesheets, both fixed by qualifying the selector: the spent stepper button kept a pointer cursor, and the form item kept the field-stack margin system/form.css gives it, which pushed the limit line a full line down and made the card taller than its neighbours.

Kernel coverage pins both halves: the card variable and the aria-describedby wiring, plus an offer that nothing caps carrying neither.