EIP-712 Payload Builder

Builds an EIP-712 payload field by field, validating each declared type and coercing values before hashing — passing a string where uint256 is expected hashes differently and produces a signature that verifies against nothing. Takes chainId from the selected network so a signature cannot accidentally be made valid everywhere. After signing it recovers the signer locally: if that does not come back as your own address, the wallet hashed something other than what was shown. After signing, the signer is recovered locally and compared to your own address. If it does not match, the wallet hashed something other than what was displayed, and the signature is useless — which is worth knowing immediately rather than when a contract rejects it later.