CHANGE_ID=LAUNDRY-20260913-POS-QTY-001
TITLE=Laundry POS quantity and cart line totals
SCOPE=UI/JS only; no migration; no database schema change
FILES=2

Changes:
- Quantity selector (+/- and direct input) in laundry service modal.
- Modal always opens in laundry mode so quantity can be selected even for single-service items.
- Identical product/service/accessories lines merge by increasing quantity.
- Cart shows quantity, unit price, and line total.
- Cart shows total piece count.
- Existing backend qty handling is reused; no financial/ZATCA logic changed.

Manual acceptance test:
1) Add Thobe / Wash qty=5 at 12 SAR => line total 60 and total pieces 5.
2) Add same Thobe / Wash qty=2 => same line becomes qty 7; total pieces 7.
3) Add same product with different service/accessories => separate row.
4) Edit row qty directly => line total and total pieces recalculate.
