vault.js: Argon2id key derivation + XSalsa20-Poly1305 encryption
via libsodium-wrappers-sumo. No raw crypto primitives.
Wallet creation now requires a password. The mnemonic or private
key is encrypted before storage — only the ciphertext blob
(salt, nonce, ciphertext) is persisted. The plaintext secret
is never stored.
Sending requires the password to decrypt the secret, derive
the signing key, and construct the transaction. Wrong password
is caught and reported.
