Pre-check token balance before pay transfer

[iexwr]

Context

If the payer's SAC balance is lower than amount, token.transfer will panic inside the contract. Surfacing this earlier gives a clearer failure.

Acceptance criteria

• Before token.transfer, read token.balance(&payer) and abort with a clear message if it is < amount.
• Add a test where the payer has insufficient balance and the call aborts before state changes.

Technical notes

• TokenClient exposes balance(client, account).
• Keep the transfer as the last mutating step so a failed pre-check leaves status unchanged.

Testing

• pay_insufficient_balance_fails test using a mock token with a low balance.