Implement shopping cart

[Copilot]

The Products page had a stubbed-out handleAddToCart that only fired an alert(). There was no cart state, no cart page, and no way to review or modify selected items.

Changes

CartContext.tsx (new)

Global cart state via React Context. Exposes addToCart, removeFromCart, updateQuantity, clearCart, totalItems, and totalPrice. All callbacks and derived values are memoized with useCallback/useMemo.

Cart.tsx (new)

Cart page at /cart with:

• Item rows: product image, name, unit price (discount strikethrough when applicable), quantity stepper, line total, remove button
• Order Summary panel with grand total and "Proceed to Checkout" CTA
• Empty state with "Browse Products" link
• Full dark mode support

Navigation.tsx

Cart icon added to the nav bar. Shows a badge with live item count (only visible when totalItems > 0), links to /cart.

Products.tsx

handleAddToCart now calls addToCart() from CartContext instead of alert().

App.tsx

Wrapped ThemedApp with <CartProvider> and registered the /cart route.

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Original prompt

implement shopping cart

Created from Copilot CLI via the copilot delegate command.