Validate: reject invalid IBAN before submit

[1nonlypiece]

Summary

No IBAN field existed anywhere in the app. Adds isValidIban() (lib/validation/iban.ts), a dependency-free ISO 13616 MOD-97-10 checksum validator (verified against well-known real IBAN test vectors from GB/DE/FR), and a new "Linked payout bank account" field in WalletSection (the natural home for anchor-withdrawal-adjacent settings) using the same reject-and-show-inline-error pattern as #1584's phone validation: an invalid IBAN shows a role="alert" error and skips triggerSave(); empty is treated as valid since the field is optional.

Also adds the settings.wallet.payout_iban_invalid translation key to en.json/es.json, same scoped approach as #1584 -- not backfilling the rest of the pre-existing missing settings.* tree.

Testing

• node node_modules/vitest/vitest.mjs run ... tests/unit/validation/iban.test.ts tests/unit/settings/WalletSection.test.tsx -- 9/9 pass (valid IBANs from 3 countries with/without spacing, case-insensitivity, empty-is-valid, bad checksum, structurally invalid, too-short, plus the component showing/clearing the error)
• npm run test:unit:vitest -- 306 passing (was 297 on main), zero regressions
• npx eslint on all touched files -- clean
• npx tsc --noEmit -- no new errors in any touched file

Closes #1472