fix(invoices): make self-billed invoices creditable and their dates visible (#1827)

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>

lib/errors/structured-errors.ts

@@ -926,6 +926,11 @@ const INVOICE: Record<string, StructuredErrorEntry> = {
     message_sv: 'Endast skickade, betalda eller förfallna fakturor kan krediteras.',
     message_en: 'Only sent, paid, or overdue invoices can be credited.',
   },
+  INVOICE_CREDIT_NO_NUMBER: {
+    httpStatus: 400,
+    message_sv: 'Ursprungsfakturan saknar fakturanummer och kan inte krediteras.',
+    message_en: 'The original invoice has no invoice number and cannot be credited.',
+  },
   INVOICE_CREDIT_ISSUE_INCOMPLETE: {
     httpStatus: 500,
     message_sv:

lib/invoices/__tests__/display.test.ts

@@ -1,5 +1,24 @@
 import { describe, it, expect } from 'vitest'
-import { isTextLikeLine } from '@/lib/invoices/display'
+import { creditConfirmNumber, isTextLikeLine } from '@/lib/invoices/display'
+
+describe('creditConfirmNumber', () => {
+  it('uses our own invoice number when present', () => {
+    expect(
+      creditConfirmNumber({ invoice_number: 'F-2026010', external_invoice_number: null }),
+    ).toBe('F-2026010')
+  })
+
+  it('falls back to the external number for self-billed invoices (issue #1820)', () => {
+    expect(
+      creditConfirmNumber({ invoice_number: null, external_invoice_number: 'SB-2026-17' }),
+    ).toBe('SB-2026-17')
+  })
+
+  it('returns null when the invoice carries no number at all', () => {
+    expect(creditConfirmNumber({ invoice_number: null, external_invoice_number: null })).toBeNull()
+    expect(creditConfirmNumber({})).toBeNull()
+  })
+})
 
 describe('isTextLikeLine', () => {
   it('is true for explicit text rows regardless of amounts', () => {

lib/invoices/display.ts

@@ -17,6 +17,21 @@ export function invoiceDisplayNumber(invoice: {
   return invoice.invoice_number ?? invoice.external_invoice_number ?? INVOICE_NUMBER_DRAFT_LABEL
 }
 
+/**
+ * The number a user must type to confirm crediting an invoice. Regular
+ * invoices confirm with their own `invoice_number`; self-billed invoices have
+ * `invoice_number` null by design, so the counterparty's
+ * `external_invoice_number` (the number shown everywhere in the UI) is the
+ * one to type. Null when the invoice carries no number at all: the credit
+ * flow must stay disabled then.
+ */
+export function creditConfirmNumber(invoice: {
+  invoice_number?: string | null
+  external_invoice_number?: string | null
+}): string | null {
+  return invoice.invoice_number ?? invoice.external_invoice_number ?? null
+}
+
 /**
  * True when an invoice line should render as a pure text row: description
  * only, no quantity/unit/price/amount columns. Explicit text rows