Refresh the "Shipping by" date in existing carts when an offer's date changes

When I update the shipping date on a preorder offer, customers who already have that product in their cart keep the old date. It carries through to checkout, the order and the confirmation email, so customers end up with an incorrect dispatch timeframe.

For my business this is a big deal. We sell wedding flowers, and our customers plan around fixed wedding dates, so a wrong dispatch month causes real confusion and extra work for my team to correct after the fact.

STOQ already stores the current date in the shipping_text metafield on each variant, and Shopify's Cart API allows line item properties to be updated on items already in the cart. It would be great if STOQ's app embed could check each cart line's "Shipping by" value against the current offer date and update it automatically when they don't match.

Ideally this would happen when the customer next visits the store or opens their cart, without removing the item or changing the selling plan.