B2B Order Documents

A B2B order accumulates paperwork (the buyer's PO, quotation, packing lists, shipping documents) and today the only place it can live is email. This adds an order Documents area: uploaded files in private storage, per-document customer visibility, listed in the buyer's order view. It also extends the dashboard's client-side print path with two generated documents the data already supports: a carton-level packing list and a payment-instructions sheet.

Numbered invoices with PDF generation and legal numbering are deliberately not part of this.

Plan: https://github.com/spree/spree/blob/main/docs/plans/6.1-b2b-order-documents.md

Tracked in Linear as V-3613.