[Feature Request] Support user trigram / initials tag in document numbering masks

Feature Request

Currently, document numbering masks (e.g. for invoices, proposals, orders) support dates ({yy}{mm}), counters ({0000}), and user ID/login ({uu} or {uuuu}). However, there is no direct support for using the user's initials/trigram (from the user card) inside the numbering mask.

In many corporate workflows, companies require quotes and invoices to include the commercial or author trigram (e.g., PROP-LM-2609-0001) rather than a numeric ID or full login.

Use case

Field involved: user.initials / user.ref_ext (or dedicated trigram field). Modules impacted: Proposal, Customer Invoice, Order numbering engines (mod_...).

Suggested implementation

Add a dedicated mask variable (e.g., {uuu} or {utr}) representing the author/sales representative's trigram or initials. Fall back gracefully (e.g., to login initials or XXX) if the trigram field is empty on the user card.