feat: add email notification for new leave requests

- mailer.ts: nodemailer transport via local sendmail
- leave-notification.ts: HTML email matching PHP template
- Sends notification to LEAVE_NOTIFY_EMAIL on new leave request
- Non-blocking: errors logged but don't fail the request
- Added LEAVE_NOTIFY_EMAIL and APP_URL env vars

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.env.example

@@ -23,6 +23,10 @@ MAX_UPLOAD_SIZE=52428800
 CONTACT_EMAIL_TO=
 CONTACT_EMAIL_FROM=
 SMTP_FROM=
+LEAVE_NOTIFY_EMAIL=
+
+# App URL (for email links)
+APP_URL=
 
 # CORS (production only, comma-separated)
 CORS_ORIGINS=