Add data export/import with encryption #124
Description
Summary
Add encrypted data export/import in v2 so users can back up and restore securely.
Requirements
- Export: serialize current dataset, add minimal metadata (version, created timestamp, compression flag), encrypt, and save as .json.
- Import: decrypt first, verify AES-GCM auth tag, decompress if flagged, validate schema/version, then load.
- Provide clear errors for wrong key, corrupted file, or version mismatch.
- Perform heavy work off the main thread; show a visible progress indicator.
Acceptance Criteria
- Users can export an encrypted .json backup and fully restore via import.
- Import rejects files that fail decryption/auth-tag verification.
- Progress/feedback is shown for long operations; UI remains responsive.
- Round-trip tests confirm data equivalence after restore.