Fixes#1175. The stored personnummer exists only as AES-256-GCM
ciphertext (+ last4), so the editor cannot rehydrate it and sent an
empty string; buildInvoiceWriteData then failed ROT/RUT validation and
every edit of a draft deduction invoice was blocked with "Personnummer
krävs för ROT/RUT-avdrag" unless the user re-entered the customer's
personnummer.
buildInvoiceWriteData accepts the stored ciphertext from the update
path: an empty field on an invoice that still has deduction lines
means keep, a typed value replaces, and removing every deduction line
clears as before. The editor hint shows the kept last4 in edit mode
(new i18n key, sv+en).
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
