Redo signup page

[harryrigg]

Sign up page should be redone to use react-hook-form instead of manually managing state and errors. This will make the code more maintainable and the form will be more accessible.

Use zod to define a schema that matches the backend requirements, and provides descriptive error messages.

See the create and edit event pages for reference.

[github-actions[bot]]

Branch issue-123-Redo_signup_page created!