Automatically sends welcome emails when new users sign up via Clerk webhooks.

Example email for new user:

1. Set SENDGRID_API_KEY environment variable
2. Update constants in main.tsx: FROM_EMAIL, FROM_NAME, EMAIL_SUBJECT and the email body in email-body.tsx
3. Get HTTP endpoint URL for main.tsx
4. Add that HTTP endpoint URL as a webhook in Clerk dashboard for user.created events