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>

messages/en.json

@@ -1330,7 +1330,13 @@
     "plusgiro_error": "Invalid plusgiro number",
     "swish_label": "Swish",
     "swish_placeholder": "123 XXX XX XX or 07X XXX XX XX",
-    "swish_error": "Invalid Swish number (business number 123XXXXXXX or mobile number 07XXXXXXXX)"
+    "swish_error": "Invalid Swish number (business number 123XXXXXXX or mobile number 07XXXXXXXX)",
+    "iban_label": "IBAN",
+    "iban_error": "Invalid IBAN (SE followed by 22 digits)",
+    "iban_hint": "Required for the payment file (ISO 20022) when paying salaries",
+    "bic_label": "BIC/SWIFT",
+    "bic_placeholder": "Filled in automatically",
+    "bic_error": "Invalid BIC/SWIFT (8 or 11 characters)"
   },
   "settings_invoice_form": {
     "default_our_reference_label": "Default \"Our reference\"",

messages/sv.json

@@ -1330,7 +1330,13 @@
     "plusgiro_error": "Ogiltigt plusgironummer",
     "swish_label": "Swish",
     "swish_placeholder": "123 XXX XX XX eller 07X XXX XX XX",
-    "swish_error": "Ogiltigt Swish-nummer (företagsnummer 123XXXXXXX eller mobilnummer 07XXXXXXXX)"
+    "swish_error": "Ogiltigt Swish-nummer (företagsnummer 123XXXXXXX eller mobilnummer 07XXXXXXXX)",
+    "iban_label": "IBAN",
+    "iban_error": "Ogiltigt IBAN (SE följt av 22 siffror)",
+    "iban_hint": "Krävs för betalfil (ISO 20022) vid löneutbetalning",
+    "bic_label": "BIC/SWIFT",
+    "bic_placeholder": "Fylls i automatiskt",
+    "bic_error": "Ogiltig BIC/SWIFT (8 eller 11 tecken)"
   },
   "settings_invoice_form": {
     "default_our_reference_label": "Standard för Vår referens",