[CART-001] Cart UI: Shopping cart drawer and item components

## Overview
Implement the shopping cart UI — a slide-in drawer that lists cart items, quantities, subtotal, and checkout CTA.

## Acceptance Criteria
- [ ] Cart drawer opens/closes from the header cart button
- [ ] Each item shows: product image emoji, name, unit price, quantity stepper (+/−), and line total
- [ ] Subtotal is calculated and displayed
- [ ] Empty cart state is shown with a "Start Shopping" CTA
- [ ] Drawer is accessible (focus trap, keyboard-closeable, ARIA roles)
- [ ] Responsive on mobile (full-screen sheet)

## Design Notes
The drawer should slide in from the right. Use the existing CSS variables (`--primary`, `--surface`, etc.) for consistency.

## Out of Scope
- Persistence to a backend (see #3 CART-003)
- Checkout flow (see CART-003 and ORD-001)

## Related
- Blocked by nothing — this is a pure UI task
- #2 depends on this issue