feat: add customer cart with expiry, shipping addresses with USPS validation, and multi-item PayPal checkout

frontend/src/cart/CartContext.tsx

@@ -0,0 +1,35 @@
+import React, { createContext, useContext, useEffect, useState, useCallback } from 'react';
+import { CartItem, fetchCart } from './cartApi';
+import { useCustomerAuth } from '../customer/CustomerAuthContext';
+
+interface CartContextValue {
+  items: CartItem[];
+  itemIds: Set<number>;
+  refresh: () => void;
+}
+
+const CartContext = createContext<CartContextValue>({ items: [], itemIds: new Set(), refresh: () => {} });
+
+export function useCart() {
+  return useContext(CartContext);
+}
+
+export function CartProvider({ children }: { children: React.ReactNode }) {
+  const [items, setItems] = useState<CartItem[]>([]);
+  const { customer } = useCustomerAuth();
+
+  const refresh = useCallback(() => {
+    if (!customer) { setItems([]); return; }
+    fetchCart().then(data => setItems(data.items)).catch(() => setItems([]));
+  }, [customer]);
+
+  useEffect(() => { refresh(); }, [refresh]);
+
+  const itemIds = new Set(items.map(i => i.item_id));
+
+  return (
+    <CartContext.Provider value={{ items, itemIds, refresh }}>
+      {children}
+    </CartContext.Provider>
+  );
+}