feat(AmountConfirm): add aria-live region for screen-reader announcements (Closes #839)

[waterWang]

Summary

Adds a polite aria-live region (via the project's existing LiveRegion component) to AmountConfirm so screen-reader users are notified of status changes.

Changes

• src/pages/AmountConfirm.tsx: Imported LiveRegion component and added srAnnouncement state. On successful confirmation, announces "Amount confirmed: $"; on mismatch, announces the expected amount.
• src/pages/AmountConfirm.test.tsx: Added 3 tests:
  • Verifies LiveRegion renders with sr-only class and aria-live="polite"
  • Verifies confirmation announcement on match
  • Verifies error announcement on mismatch

Acceptance Criteria

• LiveRegion with sr-only class is rendered inside the form
• aria-live="polite" ensures SR announcements don't interrupt the user
• Confirmation success announces the confirmed amount
• Error state announces the mismatch and expected amount
• All 10 tests pass (7 existing + 3 new)

Closes #839