Line-level discounts and surcharges

Added support for modeling Peppol line discounts and surcharges, including correct UBL mapping and totals.

We’ve improved how individual invoice and credit note lines can express discounts and surcharges.

Line discounts & surcharges: Document lines can now include structured discounts and surcharges with reason codes/text and amounts, following Peppol codelists.
Parsing, XML & totals: These values round‑trip through the UBL XML (via cac:AllowanceCharge) and are taken into account when calculating line and document totals.
PDF preview: The billing document template now shows per‑line discounts (in red) and surcharges (in green) in the rendered preview.

This makes line pricing in your Peppol documents more accurate and transparent, especially when you use discounts or extra charges per line.