[FIX] Persist cart state to localStorage on page refresh

[Siddh2024]

Description

Fixes #268

The cart state was previously held only in memory and lost on page refresh. This PR adds localStorage persistence so cart items survive page reloads.

Changes Made

CartContext (src/context/CartContext.jsx)

• Created CartContext with localStorage persistence using 'restrohub-cart' key
• Provides addItem, removeItem, updateQuantity, clearCart operations
• Exposes itemCount and totalAmount derived values
• Auto-saves to localStorage on every state change
• On mount, restores cart from localStorage
• Uses useRef hydration guard to prevent SSR issues

CartDrawer (src/components/customer/CartDrawer.jsx)

• Slide-in drawer showing cart items with name, price, quantity controls
• Quantity increment/decrement and remove buttons per item
• Clear cart and total display
• Empty state with helpful message
• Overlay backdrop closes drawer on click

Navigation & MenuSection Updates

• Added cart button with SVG icon and item count badge to Navigation
• Added "Add to Cart" button on each menu item in MenuSection
• Wrapped CustomerLayout with CartProvider

Testing

1. Open restaurant menu page
2. Add items to cart via "Add to Cart" buttons
3. Open cart drawer via cart button in navigation
4. Verify items, quantities, total display correctly
5. Refresh the page - cart items should persist from localStorage
6. Clear cart - items should be removed from localStorage