Refactor user client

[alexex10]

Changes

• UserClient replaced by OnboardingClient, which accepts an account address and a sign-message callback instead of a raw L1 private key.
• onboard_subaccount error handling has changed. Previously, it silently recovered an existing sub-account (HTTP 409) by fetching it from get_accounts(). Now it raises ValidationError on conflict. Handle duplicates explicitly if you relied on the automatic recovery.
• Fixes vault_module reads starknet_domain from EndpointsConfig, but it lives on SigningConfig. AttributeError on every vault deposit/withdraw. #99.