POS variant selection now shows SKU, price, and stock before adding to cart

Overview

When selling products with variants, the POS interface now lists each available variant with its SKU, barcode, price, and current stock level before an item is added to the cart. This allows cashiers to choose the exact variant needed, rather than a potentially ambiguous parent product.

The variant selector presents all combinations, including those with zero stock. This visibility improves inventory awareness while still allowing cashiers to proceed if they select a variant with no stock.