Handle an empty wallet

{
  "error": {
    "type": "insufficient_credits",
    "message": "このアプリのWallet残高が不足しています",
    "balance": "0.00",
    "top_up_url": "https://tokengift.ai/fuel/my-app"
  }
}

try {
  const res = await client.chat.completions.create({ ... });
} catch (err) {
  if (err.status === 402) {
    // 残高切れ: ユーザーにチャージページを見せる
    showBanner({
      message: "This app is out of AI fuel.",
      cta: { label: "⚡ Add fuel", href: err.error?.top_up_url },
    });
    return;
  }
  throw err;
}