next-safe-action Validation Errors

next-safe-action Validation Errors: 1. **Schema validation** — automatic when input doesn't match `.inputSchema()` 2. **Manual validation** — via `returnValidationErrors()` in server code (e.g., "email already taken")

This skill describes how to implement validation errors in `next-safe-action` using `zod` for schema validation and `returnValidationErrors` for manual server-side checks. It demonstrates how `returnValidationErrors` throws an error and provides examples of database interactions (`db.user.findByEmail`) for business logic validation.