Also enables zeroize's `serde` feature so Zeroizing<String> can
round-trip through serde_json.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
derive_master_key now:
- length-prefixes passphrase and image_secret to eliminate concatenation
ambiguity (H1)
- normalizes passphrase to UTF-8 NFC before hashing
- returns Zeroizing<[u8; 32]> so the master key is wiped on drop (H2)
- wraps the intermediate password buffer in Zeroizing for the same reason
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
The getrandom crate (transitive dep via rand/argon2) requires the
"js" feature flag to compile for wasm32-unknown-unknown targets.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
