feat(kms): enhance onboard page with site name, chain info, and k256 pubkey

[Leechael]

Summary

• Add configurable site_name, eth_rpc_url, and kms_contract_address to [core.onboard] config, displayed on the onboard page for operator visibility
• Return k256_pubkey in OnboardResponse so it can be compared with the on-chain kmsInfo.k256Pubkey after onboarding

Config Example

[core.onboard]
site_name = "My KMS"
eth_rpc_url = "https://rpc.example.com"
kms_contract_address = "0xYourKmsContractAddress"

Test plan

• Deploy KMS with site_name, eth_rpc_url, kms_contract_address set in config
• Verify onboard page shows custom site name in the title
• Verify ETH RPC URL and KMS Contract are displayed in the attestation info section
• Perform onboard and verify k256Pubkey is shown in the result
• Compare displayed k256Pubkey with on-chain kmsInfo.k256Pubkey
• Verify page works correctly when config fields are empty (default)