Optimize CartContext: memoize derived values, stable action callbacks, and add loading state

[Samyukta04]

What

• Improves performance and user experience by memoizing expensive cart calculations and stabilizing action callbacks.
• Adds a loading flag to provide clear UI feedback during cart operations and order submission.

##Changes

1. CartContext.jsx

• Switched to useReducer for items state (existing).
• Added useState(loading) and a simulateDelay helper.
• Wrapped addItem, removeItem, and clearCart with useCallback (returning promises).
• Used useMemo for totalQuantity and totalAmount.
• Exposed contextValue via useMemo, including items, callbacks, loading, and totals.

2. OrderForm.jsx

• Integrated CartContext.loading and local isLoading to disable inputs/buttons.
• Mapped customer.postal → "postal-code" for backend compatibility.
• Read server response message and displayed it through SuccessModal.

3. SuccessModal.jsx

• Added a short loading spinner/message before showing the final success message.
• Included small optional debug logs to verify memoization (removable later)

[Aneeshie]

There are merge conflicts please resolve them.

(there are changes in main branch do your changes on top of them.)

[Samyukta04]

resolved them