feat: add React Email templates for remaining auth events

Add pre-built templates for:
- account-created: Welcome email with account details
- two-factor-enabled: 2FA enabled notification (green success styling)
- two-factor-disabled: 2FA disabled warning (amber warning styling)

All templates follow the existing design system and include:
- Responsive layout
- Customizable props (appName, logoUrl, supportEmail, etc.)
- Preview props for email development

Author: R44VC0RP