Parse existing Invoice_*.pdf files (PyMuPDF) for the invoice number and
"Work performed" period, then default the next period to the 14 days
after the latest one, the number to <month><letter>/<year> with the
letter incrementing within the month, and the amount to 7250 EUR.
Also add an optional days-off deduction at the derived daily rate and
ask before overwriting an existing PDF.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
