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>

lib/api/schemas.ts

@@ -1363,8 +1363,8 @@ export const UpdateSettingsSchema = z.object({
     )
     .nullable()
     .optional(),
-  iban: z.string().optional(),
-  bic: z.string().optional(),
+  iban: z.string().regex(/^SE\d{22}$/, 'Ogiltigt IBAN (SE följt av 22 siffror)').nullable().optional().or(z.literal('')),
+  bic: z.string().regex(/^[A-Z]{6}[A-Z0-9]{2}([A-Z0-9]{3})?$/, 'Ogiltig BIC/SWIFT (8 eller 11 tecken)').nullable().optional().or(z.literal('')),
   accounting_method: AccountingMethodSchema.optional(),
   invoice_prefix: z.string().nullable().optional(),
   next_invoice_number: z.number().int().positive().optional(),