feat(saleor): Phase 3 - Cart Migration

- Create Saleor checkout store (Zustand + persist)
- Update CartDrawer to use Saleor checkout
- Update Header to use Saleor checkout store
- Update ProductDetail with Add to Cart functionality
- Add checkout initialization on app load
- Handle checkout line add/update/delete operations
- Add error handling and loading states

src/components/layout/Header.tsx

@@ -1,19 +1,24 @@
 "use client";
 
-import { useState } from "react";
+import { useState, useEffect } from "react";
 import Link from "next/link";
 import Image from "next/image";
 import { AnimatePresence } from "framer-motion";
-import { useCartStore } from "@/stores/cartStore";
+import { useSaleorCheckoutStore } from "@/stores/saleorCheckoutStore";
 import { User, ShoppingBag, Menu } from "lucide-react";
 import MobileMenu from "./MobileMenu";
 import CartDrawer from "@/components/cart/CartDrawer";
 
 export default function Header() {
   const [mobileMenuOpen, setMobileMenuOpen] = useState(false);
-  const { items, toggleCart } = useCartStore();
+  const { getLineCount, toggleCart, initCheckout } = useSaleorCheckoutStore();
 
-  const itemCount = items.reduce((count, item) => count + item.quantity, 0);
+  const itemCount = getLineCount();
+
+  // Initialize checkout on mount
+  useEffect(() => {
+    initCheckout();
+  }, [initCheckout]);
 
   return (
     <>