Build Input + ErrorMessage shared components

## Goal
Build reusable `Input` and `ErrorMessage` components with label, touched state, and error styling for form fields.

## Acceptance criteria
- [ ] `Input` accepts label, value, onChange, error, touched
- [ ] Error shows only when `touched && error`
- [ ] `ErrorMessage` renders accessibly (e.g., `aria-live="polite"`)
- [ ] Components used in at least one form

No files or tests are named, so first locate the existing form entry point and current component conventions. Build reusable Input and ErrorMessage components, use them in at least one form, and verify the listed props, touched-and-error display rule, and polite accessible error announcement.

Written by the indexing model from the issue text.