Implemented promocode application feature in the cart, allowing users to select and apply discounts during checkout. Updated GraphQL mutation, cart logic, and UI to support this functionality.
- Enhanced `cart.vue` with a new promocode selection section, including dropdown and styling.
- Modified `buyOrder` mutation to accept `promocodeUuid` and `forceBalance` parameters.
- Updated translations (`en-gb.json` and `ru-ru.json`) to include promocode-related strings.
Improves user experience by enabling discount application directly in the cart. No breaking changes.
