USER STORY: Discount codes during checkout

[barkode]

As a Shopper, I want to be able to apply discount codes during checkout so that I can receive discounts on my purchases.

Acceptance Criteria:

• Users can enter a discount code and see an updated total if valid.
• Invalid codes display an error message.
• Only one discount code can be applied per order.

Tasks:

• Add a discount code input field at checkout.
• Implement validation for discount codes, showing applicable discount.