Sticky Cart Backorder Button Integration

My site already shows a sticky cart panel, and I need a small but precise upgrade: whenever a product is marked “Backorder,” a dedicated Backorder button must appear inside that same sticky panel.

What the button must do
• On click it should launch WhatsApp (mobile or web) with a pre-filled chat message that contains the product name, SKU and quantity, so customers can send the request directly to us. The intent is not to add the item to the cart or show an on-site message; it is purely a WhatsApp hand-off.

Core logic I expect
• Detect backorder status for each product and conditionally render the button only in those cases.
• Keep the existing Add-to-Cart behaviour untouched for regular stock items.
• Place the new control neatly within the current sticky cart HTML so it inherits the panel’s styling.

Deliverable
A clean, commented code snippet (JS/CSS and any back-end hook you deem necessary) plus brief installation notes. After I drop it into my theme or code base, it should work immediately across desktop, iOS and Android browsers without breaking the current checkout flow.