Enhance checkout and download functionality for guest users

Commit notes

This commit introduces several improvements to the checkout and download processes, including: - Added guest checkout functionality, allowing users to purchase without an account. - Implemented email validation for guest purchases and ensured proper handling of guest purchase associations. - Enhanced the download experience by providing a zip file containing all samples in a pack for guest users. - Updated API routes and client methods to support the new guest checkout flow. - Removed legacy Stripe references and ensured all payment processes are aligned with the Mollie integration.

These changes streamline the user experience for guest purchases and improve overall functionality.