Surfaces the pre-migration SREF backup to the user so they can roll back a
category migration without digging into the filesystem:
- 90-day dismissable banner at the top of Settings > Categories pointing to
the automatic backup (hidden once reverted, once dismissed, or past 90d).
- Permanent "Restore a backup" entry in Settings > Categories, available as
long as a migration journal exists (even past the 90-day window).
- Confirmation modal with two-step consent, red Restore button, fallback
file picker when the recorded path is missing, PIN prompt for encrypted
SREF files, full-page reload on success.
Internals:
- New `categoryRestoreService` wrapping `read_import_file` +
`importTransactionsWithCategories` with stable error codes
(file_missing, read_failed, parse_failed, wrong_envelope_type,
needs_password, wrong_password, import_failed).
- New `file_exists` Tauri command for the pre-flight presence check.
- On success: `categories_schema_version=v2` + merge `reverted_at` into
`last_categories_migration`.
- Pure `shouldShowBanner` / `isWithinBannerWindow` helpers with tests.
- FR/EN i18n keys under `settings.categoriesCard.restore*`.
- CHANGELOG entries in both locales.
Closes#122
