Fixed-price quantity discount tiers (ignore Shopify price changes)

Allow merchants to lock quantity discount tier prices so they remain fixed regardless of changes to the product's Shopify price or compare-at-price. Currently, quantity discounts are calculated based on the current product price, so when a merchant runs a sale (e.g., -50%), the quantity discount tiers shift too. Merchants need the option to set absolute/fixed tier prices that stay unchanged even when the base product price is modified.