Refactor: Integrate Mollie payment gateway and enhance checkout functionality

Commit notes

This commit further integrates the Mollie payment gateway by refining the checkout process. Key changes include: - Improved initialization of the Mollie client with error handling. - Added validation to prevent duplicate purchases for logged-in users. - Updated payment creation and retrieval methods to align with Mollie's API. - Removed Stripe-related code and adjusted pack management to reflect the new payment structure.

These enhancements streamline the checkout experience and ensure better handling of user purchases.