Harden RefundProtocol refund accounting and token transfers

[kutluhaneth46]

Summary

• reject refunds after a payment has been fully withdrawn and refund only the remaining amount after partial withdrawal
• use SafeERC20 for all token transfers
• retain releaseTimestamp for compatibility while documenting intentional immediate withdrawal behavior
• add focused Foundry regression coverage for recipient and arbiter refunds, partial refunds, and false-returning tokens

Testing

• forge test -vvv (4 passed)

Fixes #40