Add cart icon and cart page for shopping cart functionality

[Copilot]

Implements shopping cart feature with a cart icon in navigation (using docs/design/cart.png) and a cart page displaying items with order summary.

Changes

Cart State Management

• CartContext.tsx - Global cart state with add/remove/update operations and total calculations

Navigation

• Cart icon with item count badge linking to /cart

Cart Page (/cart)

• Items table: S.No, Product Image, Name, Unit Price, Quantity, Total, Remove
• Order Summary: Subtotal, Discount (5% coupon), Shipping ($10), Grand Total
• Coupon code input, Update Cart, Proceed To Checkout buttons

Products Integration

• "Add to Cart" now adds items to cart context instead of alert

Screenshots

Cart Icon with Badge

Cart Page with Items

Original prompt

implement the simple cart icon using image from docs/design/cart.png and cart page that displays the items in the cart

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