Cart should validate product availability before checkout

[manumokalab]

The cart page currently does not validate whether the products listed are actually available for sale (e.g. in stock or saleable). There is no visual indication when a product is out of stock, and the user can proceed to checkout even if some items are unavailable.

This missing validation affects the shopping experience and can lead to failed checkouts or confusion when users try to purchase items that are no longer available.

This behavior suggests a missing feature rather than a technical malfunction, and should be addressed to ensure reliability and trust during the checkout process.