Export and import of embedded file contents (Attachments)

Problem: Current export only saves attachment metadata (id, name). File content is missing, making backups incomplete.

"attachments": [
  {
    "id": "00000000-3b5a-4638-8b01-3ddb93f974d0",
    "name": "recovery.txt"
  }
]

Goal: Enable full export & import of attachment

Export: Include attachments toggle. Reads files → Base64 + SHA-256 → JSON → encrypt. Limits: 10 MB/file, 50 MB total.

Import: Decodes Base64, verifies sha256, restores to local storage.