On a sale's detail page, unpaid sales (retail or restaurant) can now be
cancelled or deleted:
- Cancel (pending only) marks it cancelled and frees its table / clears it from
the kitchen (PosSaleService::cancelSale).
- Delete removes the sale and cascades its lines, modifiers and payments, freeing
the table first (deleteSale). Paid sales are protected — they can't be deleted.
Status badge now distinguishes cancelled/failed. Covered by PosRestaurantTest.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>