feat(email): Integrate React Email with Nodemailer/Resend

Commit notes

Set up email infrastructure with environment-based provider switching: - Nodemailer for development (local SMTP like Mailpit) - Resend for production

Added: - Email service (backend/src/services/email.ts) with dual provider support - Base email layout components (backend/src/emails/components/layout.tsx) - Welcome email template (backend/src/emails/templates/welcome.tsx) - Email index with helper functions (backend/src/emails/index.ts) - Email configuration to environment schema - React Email dev script for template preview

Dependencies added: - @react-email/components, react, react-dom - resend (production provider) - nodemailer (development provider) - react-email (dev preview tool)

TypeScript config updated for JSX support (react-jsx).

Co-authored-by: armin.naimi <[email redacted]>