Refactor: Change createdAt type from string to Date

[coderabbitai]

Description

Currently, the createdAt field in the Product type is defined as a string. This can lead to comparison issues during sorting operations.

Changes needed

1. Change the type of createdAt from string to Date in src/types/product.ts
2. Update the sortProducts function in src/helpers/sortProducts.ts to handle Date objects correctly
3. Update any related tests and type definitions

Context

• PR: 35 add sorting to the shop page #156
• Discussion: 35 add sorting to the shop page #156 (comment)

Impact

This change will improve type safety and make date comparisons more reliable during sorting operations.