Error messages specify exact requirement

A validation error that says "Invalid input" tells the user nothing about what to change. In a financial form, that friction causes abandonment — and for accessibility, WCAG 2.2 SC 3.3.1 (Error Identification) and SC 3.3.3 (Error Suggestion) require that errors both identify what is wrong and suggest a correction. "Amount must be between $0.01 and $9,999.99" passes both; "Validation failed" fails both. Generic messages also force users to guess which constraint was violated, producing support tickets that cost real money.

Define explicit, constraint-naming error strings for every financial validation rule in src/lib/validationErrors.ts or inline in the Zod schema.

// src/lib/schemas.ts
export const transferSchema = z.object({
  amountCents: z.number()
    .int('Amount must be a whole number of cents')
    .min(1, 'Minimum transaction is $0.01')
    .max(99999, 'Maximum transaction is $999.99'),
  routingNumber: z.string()
    .regex(/^\d{9}$/, 'Routing number must be exactly 9 digits')
    .refine(isValidABARoutingNumber, 'Invalid routing number — checksum failed'),
});

Return the Zod issues[0].message string directly in the API error response so the client can display it verbatim without mapping.