Exchange credentials for a JWT.

Supports two authentication methods:

1. Fingerprint: {"fingerprint": "your-fingerprint"}
2. Hotkey signature: {"hotkey": "5...", "signature": "hex...", "nonce": "uuid"}

For hotkey auth, first call GET /users/login/nonce to get a nonce, sign it with your hotkey (e.g., btcli w sign --message <nonce>), then submit the hotkey, signature, and nonce.