RAYXPRESS SHIPMENT + CLIENT DASHBOARD UPDATE

1. Client dashboard now shows only the latest shipment assigned to the logged-in client.
2. The dashboard shows the shipment/tracking number and a professional Track Shipment button.
3. Track Shipment opens pages/tracking.php using the exact tracking number.
4. Creating a shipment immediately attempts to send the customer notification through the existing PHPMailer queue system.
5. Publishing a tracking event immediately attempts to send the customer a branded email containing:
   - client name
   - tracking number
   - new status
   - current location
   - destination
   - update description
   - Track Shipment link
6. Shipment detail changes also send a notification when a customer email is available.
7. If immediate delivery fails, the message remains in email_queue for automatic retry.
8. No new SQL migration is required.
9. Existing SMTP, PHPMailer, OTP and password-mail systems are preserved.
