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/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', () => {