feat(frontend): add ARIA live-region announcements to BankDetailsModal (#1283)

## Summary
- add polite live-region announcements for modal step changes in BankDetailsModal
- add regression tests covering the new announcement behavior

## Validation
- pnpm vitest run src/components/__tests__/BankDetailsModal.a11y.test.tsx
- pnpm typecheck
- pnpm lint

Closes #1178

Author: bamiebot-maker