Fix USD class transfer typed-data signing

[kfastov]

Summary

• replace the deprecated SpotUser/ClassTransfer payload with the UsdClassTransfer typed-data schema expected by the API
• sign the action via sign_typed_data and post it as Actions::UsdClassTransfer
• surface the Hyperliquid chain metadata and reuse the typed-data nonce when submitting the request

Problem

Running cargo run --bin class_transfer (or calling ExchangeClient::class_transfer) against master returns a 422 from Hyperliquid:

Client error: status code: 422, error code: None, error message: Failed to deserialize the JSON body into the target type, error data: Some("expected value at line 1 column 1")

Hyperliquid migrated USD class transfers away from the SpotUser L1 action; the SDK is still sending that outdated payload, so the exchange rejects it before signature verification.

Testing

• cargo test
• RUST_LOG=info cargo run --bin class_transfer (now reaches the exchange and fails with the expected "Insufficient balance for withdrawal" response because the demo wallet is empty)