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>

components/invoices/__tests__/invoice-editor-flow.test.ts

@@ -173,13 +173,30 @@ function chipsInput(overrides: Partial<ForvalChipsInput> = {}): ForvalChipsInput
 }
 
 describe('deriveForvalChips', () => {
-  it('shows only currency and due terms for an all-default invoice', () => {
+  it('shows currency, invoice date and due terms for an all-default invoice', () => {
     expect(deriveForvalChips(chipsInput())).toEqual([
       { kind: 'currency', currency: 'SEK' },
+      { kind: 'invoice_date', date: '2026-08-17' },
       { kind: 'due_days', days: 30, date: '2026-09-16' },
     ])
   })
 
+  // Issue #1820: the invoice date silently defaulted to today inside the
+  // collapsed Förval panel; the chip makes the value visible in every mode.
+  it('surfaces the invoice date as a chip, and skips it while unset', () => {
+    expect(deriveForvalChips(chipsInput())).toContainEqual({
+      kind: 'invoice_date',
+      date: '2026-08-17',
+    })
+    expect(deriveForvalChips(chipsInput({ isSelfBilled: true }))).toContainEqual({
+      kind: 'invoice_date',
+      date: '2026-08-17',
+    })
+    expect(
+      deriveForvalChips(chipsInput({ invoiceDate: '' })).find((c) => c.kind === 'invoice_date'),
+    ).toBeUndefined()
+  })
+
   it('surfaces a deviating document type first', () => {
     expect(deriveForvalChips(chipsInput({ documentType: 'proforma' }))[0]).toEqual({
       kind: 'doc_type',
@@ -225,7 +242,7 @@ describe('deriveForvalChips', () => {
     })
   })
 
-  it('reduces to currency, due and received for self-billed mode', () => {
+  it('reduces to currency, invoice date, due and received for self-billed mode', () => {
     const chips = deriveForvalChips(
       chipsInput({
         isSelfBilled: true,
@@ -239,6 +256,7 @@ describe('deriveForvalChips', () => {
     )
     expect(chips).toEqual([
       { kind: 'currency', currency: 'SEK' },
+      { kind: 'invoice_date', date: '2026-08-17' },
       { kind: 'due_days', days: 30, date: '2026-09-16' },
       { kind: 'received', date: '2026-08-15' },
     ])