Add export (JSON/CSV) and import (full replace) to the Settings page.
Export supports 3 modes (transactions+categories, transactions only,
categories only) with optional password encryption using Argon2id key
derivation. Import detects encrypted .sref files, prompts for password,
and shows a destructive confirmation modal before replacing data.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
