fix: stop the public-transfer diagnostic from silently doing nothing

Confirmed live today: a real MetaMask deploy through the browser landed on
mainnet (0x06c90d9b...887a9, class 0x697437b25b8..., matches the factory's own
get_expected_account_address for that EVM owner bit for bit).

Clicking through to the optional step-5 diagnostic exposed a real bug: with no
NEXT_PUBLIC_MOROK_TREASURY_MAINNET_ADDRESS configured, `recipient` is empty,
so prepareTransfer's early-return left the button doing nothing with zero
feedback. It now disables and labels itself "No mainnet treasury configured"
instead. Also missed a hardcoded "Sepolia" in this button's label during the
network-aware pass - fixed alongside it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>