A self-billed invoice has invoice_number null by design (the counterparty's
number lives in external_invoice_number), which broke the whole credit flow:
the confirm input was disabled and compared against null, the API minted the
literal number 'KR-null', and the credit-note PDF dropped its ML 17 kap 22
reference to the original. The editor also hid fakturadatum inside the
collapsed Forval panel, so self-billed invoices silently registered with
today's date and, being immutable, could not be corrected.
- creditConfirmNumber() falls back to external_invoice_number; the credit
page uses it for reason default, subtitle, original row, preview, confirm
label/placeholder/disabled, mismatch check and submit gate
- createCreditNote numbers 'KR-<external>' for self-billed originals and
refuses with typed 400 INVOICE_CREDIT_NO_NUMBER when no number exists
- mark-sent and send select external_invoice_number and fall back for the
credit-note PDF's reference to the original
- the Forval chip line now shows the invoice date in every mode, and
self-billed mode renders fakturadatum + mottagningsdatum uncollapsed as
transcription fields next to the external number
Fixes#1820
Claude-Session: https://claude.ai/code/session_01SyDuePXxUFowaPBKpAv8SF
Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
