Creates a new user account
Client type determines how refresh tokens are returned:
web, mobile, desktop User created successfully
JWT authentication token (null if email verification required)
CSRF token for use with refresh endpoint (web clients only, null if email verification required)
Refresh token for mobile/desktop clients (null for web clients or if email verification required)
Whether email verification is required before login
Optional URL to redirect user after registration (only present if email verification not required)