Cart entries (drink lines and Pfand-zurück) now have +/- buttons next
to the quantity instead of only accumulating via repeated taps;
decrementing to 0 removes the line. Long-pressing a drink tile opens
a small overlay with buttons 1-5 to add that many at once — a plain
tap still adds one. The long-press timer is cancelled on pointerup/
leave/cancel, and the click that follows a fired long-press is
swallowed so it doesn't also add a plain 1x.
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