app.post("/transfer", async (req, res) => {
  const { fromId, toId, amount } = req.body;
  if (!fromId || !toId || !amount) {
    const err = new Error("Missing required fields: fromId, toId, amount");
    err.status = 400;
    throw err;
  }
  if (amount <= 0) {
    const err = new Error("Transfer amount must be positive");
    err.status = 400;
    throw err;
  }
  const accounts = await getAccounts();
  const sender = accounts.find((a) => a.id === fromId);
  if (!sender) {
    const err = new Error("Sender account not found");
    err.status = 404;
    throw err;
  }
  const recipient = accounts.find((a) => a.id === toId);
  if (!recipient) {
    const err = new Error("Recipient account not found");
    err.status = 404;
    throw err;
  }
  if (sender.balance < amount) {
    const err = new Error("Insufficient funds");
    err.status = 409;
    throw err;
  }
  sender.balance -= amount;
  recipient.balance += amount;
  await saveAccounts(accounts);
  res.json({
    message: "Transfer successful",
    senderName: sender.owner,
    recipientName: recipient.owner,
    amountTransferred: amount,
    senderNewBalance: sender.balance,
    recipientNewBalance: recipient.balance,
  });
});