Support invoice-level Buyer Reference (BT-10) with project and client defaults

Please add a Buyer Reference (BT-10) field that can be edited and stored individually for each invoice, with optional defaults from the associated project or client. In the EN16931/CII export examined, BT-10 is currently populated from the client's Routing-ID. This does not accommodate customers who require different references for different projects or invoices. Use Case We issue invoices for multiple construction projects belonging to the same customer. The customer's legal name and billing address remain unchanged, but different projects require different internal processing group references, for example: • Project A: zrd123 • Project B: zrd345 Public-sector customers may likewise specify different Leitweg-IDs for different receiving units or assignments. Currently, changing the client's Routing-ID affects the source used for subsequent XML generation. Creating duplicate clients or repeatedly changing client details should not be necessar…