Min transaction amount enforced with clear error

Accepting zero-amount or sub-minimum transactions creates two concrete problems: payment processors (Stripe, Adyen) reject amounts below their floor and return errors that must be handled; and zero-amount transactions that do commit pollute the ledger with noise records that complicate reconciliation. OWASP A03 covers this as inadequate input validation. The error message matters too — WCAG 2.2 SC 3.3.1 and 3.3.3 require error messages to identify the constraint, not just report failure, so users know what to correct.

Enforce a minimum of 1 cent server-side and return a message that states the actual threshold, not a generic failure string.

// src/app/api/transfers/route.ts
const schema = z.object({
  amountCents: z.number().int().min(1, 'Minimum transaction amount is $0.01'),
});
const parsed = schema.safeParse(await req.json());
if (!parsed.success) {
  return Response.json(
    { error: parsed.error.issues[0]?.message },
    { status: 422 }
  );
}

Mirror the constraint in src/components/TransferForm.tsx with min="0.01" on the amount input and a visible hint below the field.