Adds full checkout flow with shipping, coupon, and payment

Implements a complete checkout experience including address selection, shipping method, coupon application and removal, and payment method selection. Refactors the cart and checkout modules for improved state management and localization. Updates localization files, enhances cart item and summary display, and ensures the checkout logic interacts with backend APIs for coupon and shipping validation.

Improves UX for order confirmation, adds animated feedback, and ensures robust error handling throughout the flow. Enables address selection to trigger API updates, and ensures cart state resets on logout for data integrity.

Relates to improved e-commerce checkout functionality.