Adds a new digital signature module that lets users sign documents directly through the website. This streamlines approvals and quotations by reducing manual paperwork and making the signing process available online.

Implementation of the website_sign module (rba -> qsm)

PAD: https://pad.odoo.com/p/r.4dr6BjYYy7WBSNKf
TASK: https://www.odoo.com/web#id=7505&view_type=form&model=project.task&menu_id=3942&action=327

This pull request updates Odoo's accounting area to use a newer internal API, with related fixes across invoices, bank statements, payments, reports, taxes, and point of sale links. For businesses, it should improve reliability and maintainability of accounting workflows, though the work-in-progress status suggests it may still need validation before rollout.

- Task: https://www.odoo.com/web/#id=9955&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.Uy4wEA7I4G4GwPaU