Line IDs and notes in documents and previews

Added support for explicit line IDs and notes, with proper UBL mapping and PDF rendering.

We’ve made line identification and free-text notes clearer for invoices and credit notes.

Explicit line IDs: Lines now support an optional id that is parsed from and written to UBL; when missing, a sequential number is still generated.
Line notes: Each line can include a free-text note, which round-trips via cbc:Note and appears in the billing PDF as italic text under the description.
Document-level note: A top-level document note is also supported and rendered as a separate section above the seller/buyer details in the PDF.

This makes it easier to match lines to external systems and to communicate extra context directly on specific lines or the whole document.