L10n DIN5008 Move Salesperson

This module extends the German DIN 5008 invoice layout used for customer invoices. When an invoice is printed or downloaded as a PDF, the report now shows extra lines with the salesperson responsible for the document.

• The salesperson's name is printed under the payment due date area, labelled "Salesperson".
• If the salesperson has a phone number recorded, a second line labelled "Phone" is added, and the number is shown as a clickable phone link in the PDF.
• These lines only appear on sales documents (customer invoices and receipts), not on supplier bills.
• If no salesperson is set on the invoice, the lines are simply left out, so nothing is shown.

The information comes from the salesperson field already stored on the invoice (invoice_user_id), so no new data entry is required: whatever user is set as salesperson on the invoice is what gets printed. The module does not add menus, buttons, settings or automated actions; it only changes the invoice report template inherited from the DIN 5008 invoice document. It touches the accounting invoice report area and relies on the standard salesperson field of invoices.