Refactor/Organise FE Codebase

[orangejacob]

Stuff to take note:

• Preventing non-signed in users from entering certain pages.
• Global Toast to show error messages.
• Loading + Error handling for Pages (Checkout, MerchaniseList, OrderHistory, OrderSummary).
• Refactor all mock data + interface in preparation of BE API end-points.
• If there's time, create a API class that intercept fetch request, and based on the env it will fetch mock data / online data.