Bug

When --pay-chains is enabled, all TXO deposits go to chain-specific balances (e.g. tbtc4). But POST /pay/.buy only draws from the generic sat balance, making it impossible to buy tokens with chain deposits.

Steps to reproduce

1. Start JSS with --pay --pay-token GSAT --pay-rate 1 --pay-chains tbtc3,tbtc4
2. Deposit a tbtc4 TXO → balance shows tbtc4: 2083219, sat: 0
3. POST /pay/.buy { amount: 100 } → Insufficient sat balance

Expected

Should be able to buy tokens using chain-specific balances, e.g.:

POST /pay/.buy { "amount": 100, "currency": "tbtc4" }

Suggested fix

In the .buy handler, accept an optional currency field. If provided and it's an enabled chain, debit from that chain's balance instead of generic sat.

const currency = body.currency || null;
const balance = getBalance(ledger, didUri, currency);
// ... debit(ledger, didUri, satCost, currency)