fix(salary): make pain.001 betalfil generatable (company IBAN + BIC) (#950)

The ISO 20022 pain.001 salary payment file could never be generated: the
route required company_settings.iban/bic, but no settings screen wrote
those columns, so every request returned 400. The specific reason was
also swallowed by getErrorMessage (isSwedishUserMessage did not know
"krävs"/"saknar"), surfacing only the generic "Förfrågan innehåller
ogiltiga uppgifter" (issue #945).

- Add IBAN + BIC inputs to Settings > Fakturering > Bankuppgifter. BIC
  auto-derives from the clearing number / bank already entered, so in
  practice only the IBAN is typed. Validated client- and server-side.
- Route requires the company IBAN (canonical debtor form every Swedish
  bank accepts) and derives the BIC, with clear actionable errors.
- Employees are unchanged: domestic clearing + account (BBAN), which is
  what Swedish payroll collects. Only the company (debtor) uses IBAN.
- getErrorMessage recognizes "krävs"/"saknar" so payment-file reasons
  surface instead of the generic 400.

Fixes #945

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

components/settings/sections/InvoicingSettingsContent.tsx

@@ -43,6 +43,8 @@ export function InvoicingSettingsContent() {
         ? formatPlusgiroNumber((formData.get('plusgiro') as string).trim())
         : null,
       swish: normaliseSwish(formData.get('swish') as string) || null,
+      iban: (formData.get('iban') as string || '').replace(/\s/g, '').toUpperCase() || null,
+      bic: (formData.get('bic') as string || '').replace(/\s/g, '').toUpperCase() || null,
       invoice_prefix: (formData.get('invoice_prefix') as string) || null,
       next_invoice_number: parseInt(formData.get('next_invoice_number') as string) || 1,
       next_arrival_number: parseInt(formData.get('next_arrival_number') as string) || 1,