Account Invoice - Discount Supplier Info Update

This module adds discount handling to the existing supplier-info update wizard that appears on supplier invoices. It is installed automatically when both the supplier-info update module and the purchase discount module are present.

In the wizard, two new columns are shown for each line: the current discount stored on the supplier's product information and the new discount taken from the invoice line. A discount column is only displayed when at least one line actually has a discount value, so the screen stays clean otherwise.

The wizard now also considers the discount when deciding whether an invoice line matches the supplier's product information. If the discount on the line differs from the supplier discount, the line is flagged as a difference to review.

When you confirm the wizard, the new discount is written back onto the supplier's product information, alongside the other updated values. The displayed current and new costs are recalculated to reflect the discount, so the cost shown is the net cost after discount.

It touches account.move.line and the supplier-info update wizard models, and adds fields to the wizard form view. No new menus are created; the changes appear inside the existing supplier invoice update wizard.