feat: Order & Checkout System - cart, checkout, order history, tracking

- 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

client/src/store/cart.ts

@@ -0,0 +1,70 @@
+import { create } from 'zustand'
+import { persist } from 'zustand/middleware'
+
+export interface CartItem {
+  product_id: string
+  name: string
+  price: number
+  quantity: number
+  image?: string
+}
+
+interface CartState {
+  items: CartItem[]
+  total: number
+  addItem: (item: CartItem) => void
+  removeItem: (product_id: string) => void
+  updateQuantity: (product_id: string, qty: number) => void
+  clearCart: () => void
+}
+
+function calcTotal(items: CartItem[]): number {
+  return items.reduce((sum, item) => sum + item.price * item.quantity, 0)
+}
+
+export const useCartStore = create<CartState>()(
+  persist(
+    (set, get) => ({
+      items: [],
+      total: 0,
+
+      addItem: (item: CartItem) => {
+        const existing = get().items.find(i => i.product_id === item.product_id)
+        let newItems: CartItem[]
+        if (existing) {
+          newItems = get().items.map(i =>
+            i.product_id === item.product_id
+              ? { ...i, quantity: i.quantity + item.quantity }
+              : i
+          )
+        } else {
+          newItems = [...get().items, item]
+        }
+        set({ items: newItems, total: calcTotal(newItems) })
+      },
+
+      removeItem: (product_id: string) => {
+        const newItems = get().items.filter(i => i.product_id !== product_id)
+        set({ items: newItems, total: calcTotal(newItems) })
+      },
+
+      updateQuantity: (product_id: string, qty: number) => {
+        if (qty <= 0) {
+          get().removeItem(product_id)
+          return
+        }
+        const newItems = get().items.map(i =>
+          i.product_id === product_id ? { ...i, quantity: qty } : i
+        )
+        set({ items: newItems, total: calcTotal(newItems) })
+      },
+
+      clearCart: () => {
+        set({ items: [], total: 0 })
+      },
+    }),
+    {
+      name: 'cart',
+    }
+  )
+)