Function write_device_privkey 

pub fn write_device_privkey(
    user_id: &str,
    device_id: &str,
    privkey_b64: &str,
) -> Result<&'static str, CredstoreError>

Persist a base64url-encoded X25519 private key for (user_id, device_id) to config.json. Always returns "plaintext".