Support automatic partial preorder quantities when ordered quantity exceeds available stock

Currently, STOQ only identifies a product as a preorder once the inventory has reached zero. This creates a limitation when a customer purchases more units than are currently in stock.

Example:

• Inventory available: 1

• Customer purchases: 2

Expected behavior:

• 1 unit should be fulfilled from available stock.

• 1 unit should be identified as a preorder.

At the moment, the entire line item is treated as a regular purchase, making it difficult for our fulfillment team to determine that part of the quantity should be handled as a preorder.

A previous app we used displayed a sub-line beneath the product in the Shopify order, such as:

Pre-ordered items: 1

This made it immediately clear how many units in the line item were purchased on preorder without needing to manually verify inventory or cross-reference the STOQ dashboard.

It would be extremely helpful if STOQ could support similar functionality by identifying and displaying the preorder quantity within a line item when an order exceeds the available inventory. This would significantly improve order processing and reduce the risk of fulfillment mistakes for merchants handling mixed stock/preorder quantities.