Refactors cart and checkout code for improved readability and consistency

This update includes several changes aimed at enhancing code clarity and maintainability:

• Removed unused imports and unnecessary code from various files, streamlining the codebase.
• Simplified string concatenation in the order confirmation screen for better readability.
• Updated constructors to use super for parameter forwarding, improving consistency across widget definitions.
• Adjusted null checks and formatting in several models and services to enhance code quality.

These changes contribute to a cleaner and more efficient codebase, facilitating future development and maintenance.