Variant prices override base price when applicable

When a variant has a custom price but the display and cart always use the base product price, customers see one price on the listing and are charged a different amount at checkout — or variants intended to cost more (e.g., XL size surcharge) are sold at the base price, creating a revenue shortfall on every sale of that variant. ISO 25010:2011 functional-correctness requires that price-affecting selections are reflected consistently across all display and calculation contexts. A product page showing $29.99 and a cart showing $24.99 erodes trust even when the error favors the customer.

Centralize variant price resolution in src/lib/pricing.ts so all display components and cart calculations use a single source of truth.

export const resolvePrice = (
  product: { price: Decimal },
  variant?: { price?: Decimal | null } | null
): Decimal => {
  if (variant?.price != null) return variant.price
  return product.price
}

Import resolvePrice in src/components/product/ProductCard.tsx, src/app/products/[slug]/page.tsx, and src/components/cart/CartItem.tsx. Do not inline the fallback logic in each component — divergence between components is how this bug reappears.