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>

DECISIONS.md

@@ -1171,4 +1171,5 @@ One line per decision: `[YYYY-MM-DD] <decision>: <why>`. Appended by agents and
 [2026-08-22] Per-company invoice sending domains are gated by a manually granted capability (custom_sender_domain), deliberately NOT in PAID_CAPABILITIES: the opt-in must not be trial-seeded or written by the Stripe subscription sync, and non-grantees must see an unchanged invoicing settings page (the section hides on the 403 capability_blocked envelope). The sending-domain module has no Resend orphan-adoption path (a name that already exists is a 409), because the same Resend account holds the platform's own outbound domain. The delivery log was left untouched (no from_address column): adding it would re-open the hardened invoice_deliveries evidence triggers/redaction paths for a nice-to-have, and the log already measures delivered/bounced per send.
 [2026-08-22] company_sending_domains verification state (domain, status, resend_domain_id, dns_records, verified_at, last_checked_at) is service-role only via a BEFORE trigger keyed on the JWT role claim; tenant JWTs may only open a pending claim and edit sender_local_part/sender_name/enabled. Skeptic refutation: RLS alone let a granted admin insert {domain: platform sender domain, status: verified} through PostgREST and send invoice mail as the platform. The claim/verify helpers therefore take a separate service-role writer for those columns. Second refutation: a domain Resend later flips to failed made every invoice send for that company fail; the Resend adapter now retries once as the platform sender when an explicit company From is rejected (nothing was sent on the rejected attempt, so the retry cannot double-send).
 [2026-08-22] Sending-domain verification writes bind by (id, company_id, domain, resend_domain_id IS NULL) and verify/webhook compare Resend's domain name with the row before writing verified; resolveInvoiceSender additionally refuses reserved platform domains and non-hostnames at send time. Skeptic re-check: a tenant could delete and re-insert its pending row under the same id with a reserved domain during the claim's Resend round-trip (TOCTOU), and the service-role writer updated by id alone. Defense in depth over a single gate.
+[2026-08-24] Issue #1820 self-billed credit fix: creditConfirmNumber()/originalRef fall back invoice_number -> external_invoice_number (typed 400 INVOICE_CREDIT_NO_NUMBER if both null) instead of relaxing the DB numbering constraint or dropping the type-the-number confirm step; the confirm step stays (dropping it is a founder call). The invoice-date Forval chip surfaces in ALL editor modes, not only self-billed: the silent today-default exists in every mode and the chip line already carries the due date. In self-billed mode fakturadatum + mottagningsdatum render uncollapsed next to the external number (transcription fields, not defaults); the panel rows are hidden there because registering the same RHF field twice desyncs the inputs. The v1 credit route's existing id-slice fallback was left unchanged (public API behavior).
 [2026-08-24] No-IBAN reconnect pairing (issue #1709) uses only per-currency exactly-one-each-side elimination, deliberately WITHOUT name equality: ASPSPs reformat product names between consents, so requiring it would silently disable the fix for the banks that need it, while the one-per-currency guard already bounds a mis-pair to skipping rows whose account+date+amount+occurrence all collide. upsertFromPsd2 needed no change: its explicit reuse_cash_account_id promote path already covers a same-connection holder, so the fix only names the paired row from the callback.