fetchJettonBalance hard-codes 9 decimals for any non-TON jetton (USDT shows 1000× too small)

[sepivip]

What happens

When <TonPayButton> is configured for a non-TON asset (e.g. asset="EQCxE6mUtQJKFnGfaROTKOt1lZbDiiX1kCixRv7Nw2Id_sDs", USDT mainnet), the "Insufficient Funds" modal shows a balance 1000× smaller than reality — a wallet with 20 USDT displays as 0.02 USDT.

Root cause

fetchJettonBalance in packages/ui-react/src/hooks/useWalletBalance.ts reads:

const decimals = wallet.jetton?.decimals ?? 9;

But the toncenter v3 /api/v3/jetton/wallets response returns wallet.jetton as the master address string, not an object:

{
  "jetton_wallets": [{
    "balance": "20000000",
    "jetton": "0:B113A994B5024A16719F69139328EB759596C38A25F59028B146FECDC3621DFE"
  }],
  "metadata": {
    "0:B113...": {
      "token_info": [{
        "extra": { "decimals": "6" }
      }]
    }
  }
}

So .decimals on a string is undefined → falls back to 9. USDT has 6 decimals → balance shown as raw / 10^9 instead of raw / 10^6.

Downstream consequence

Any merchant integrating USDT sees users wrongly routed to the "Top up with crypto" view. Users who pay via that QR flow bypass the handlePay callback — so the merchant's reference never gets attached, the transfer completes on-chain, but the order stays pending forever (pay.ton.org only indexes transfers that were built through the SDK handshake with a reference).

Suggested fix

Resolve decimals from metadata[masterAddr].token_info[0].extra.decimals (where toncenter v3 actually puts them), with a fallback for the legacy shape in case the response evolves:

let decimals = 9;
const masterAddr = typeof wallet.jetton === "string" ? wallet.jetton : null;
const fromMetadata = masterAddr
  ? data?.metadata?.[masterAddr]?.token_info?.[0]?.extra?.decimals
  : undefined;
if (fromMetadata != null) {
  const parsed = Number(fromMetadata);
  if (Number.isFinite(parsed)) decimals = parsed;
} else if (typeof wallet.jetton?.decimals === "number") {
  decimals = wallet.jetton.decimals;
}

Affects every jetton whose decimals ≠ 9. Version observed: @ton-pay/ui-react@0.3.2.

Happy to open a PR if useful.