Fix negative mintedValue passing through zero-check in fee loop

The guard at the top of the inner fee loop only checked
mintedValue == BigInt.zero, but mintedValue can go negative when
nFeeRet (starting from minFeeForGroup on retry) exceeds the UTXO
group's total value. A negative mintedValue would flow into
createSparkMintRecipients causing invalid outputs.

Change the check to <= BigInt.zero to match the intent of the
commented-out MoneyRange check from the C++ reference implementation.

https://claude.ai/code/session_01D9ssjMkQAMoCUcrPzVVtEq

Author: claude

lib/wallets/wallet/wallet_mixin_interfaces/spark_interface.dart

@@ -1653,7 +1653,7 @@ mixin SparkInterface<T extends ElectrumXCurrencyInterface>
         }
 
         // if (!MoneyRange(mintedValue) || mintedValue == 0) {
-        if (mintedValue == BigInt.zero) {
+        if (mintedValue <= BigInt.zero) {
           valueAndUTXOs.remove(itr);
           skipCoin = true;
           break;