Add email sending step (SMTP)

**Description:**
Send emails via SMTP with templates and attachments.

**Tasks:**
- [ ] Design Email step type
- [ ] Integrate nodemailer
- [ ] Add SMTP configuration
- [ ] Support HTML templates
- [ ] Handle attachments
- [ ] Add template variables
- [ ] Test with major providers

**Acceptance Criteria:**
- Emails sent successfully
- Templates work
- Attachments included
- Provider compatibility