feat(core): length-prefixed Argon2 input + NFC + Zeroize (audit H1, H2)

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>

crates/idfoto-cli/Cargo.toml

@@ -20,3 +20,4 @@ ed25519-dalek = { version = "2", features = ["rand_core"] }
 rand = "0.8"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+zeroize = "1"