Add a comprehensive one-page custom checkout experience

Create a new order.php file for a multi-step checkout process, integrating with WHMCS API for product fetching, client creation, and order placement. Update config.php with WHMCS API credentials and product IDs.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 349f2130-a169-4fda-8f2a-f3be20b651f6
Replit-Commit-Checkpoint-Type: full_checkpoint
Replit-Commit-Event-Id: 8460942a-a266-4919-9a07-24d6efbc6181
Replit-Helium-Checkpoint-Created: true
