hdWalletFromXprv() and getSignerForAddress() for xprv type were deriving
addresses directly from the root key (m/N) instead of the standard BIP44
Ethereum path (m/44'/60'/0'/0/N). This caused imported xprv wallets to
generate completely wrong addresses.
Navigate to the BIP44 Ethereum derivation path before deriving child
addresses, matching the behavior of mnemonic-based wallet imports.