Improvement of the checkout page

Main task:
- Add the ability to select the number of payments (selects in payment methods).
- Transfer selected data via the payment gateway to the payment page.
- On the product page — implement a popup with price calculation for various payment methods.
- Cross-tab scripting, session caching, etc. are applied.

Additional (for other tasks in checkout):
- Redesign of the checkout page.
- Transfer of delivery methods.
- Adding pickup functionality, including the choice of pickup points.