- Backend: Add GET /api/orders/:id, PUT /api/orders/:id/cancel, GET /api/orders/:id/tracking with mock Yamato data
- Frontend: Zustand cart store with localStorage persistence (key 'cart')
- Frontend: CartPage (/cart) with quantity controls, item removal, JPY totals
- Frontend: CheckoutPage (/checkout) with shipping address form, auth guard, POST to /api/orders
- Frontend: OrderHistoryPage (/account/orders) with Japanese status labels, auth guard
- Frontend: OrderDetailPage (/account/orders/:id) with cancel button, tracking section, auth guard
- Updated App.tsx with all four new routes
- Updated ProductDetail.tsx to use cart store with View Cart link after adding
- Updated Navbar.tsx with cart icon badge (item count) and 注文履歴 order history link
