Encryption Improvements
- .encrypted extension for encrypted files: Encrypted files now use .encrypted extension (e.g., note.md → note.md.encrypted) for clear identification. Decryption removes the extension automatically. Existing files without the extension remain compatible. Fix cross-platform line ending handling: \r\n (Windows) line endings caused encryption detection and frontmatter parsing to fail. Now normalizes to \n before processing.
- Prevent double-encryption: Guard against encrypting already-encrypted content.