fix: shipping cost calculation and performance optimization

- Fix shipping cost not included in checkout total
- Add useShippingMethodSelector hook for proper abstraction
- Remove blocking initCheckout from Header for better performance
- Checkout now initializes lazily when cart opens or item added

src/lib/hooks/useShippingMethodSelector.ts

@@ -0,0 +1,73 @@
+"use client";
+
+import { useCallback } from "react";
+import { createCheckoutService } from "@/lib/services/checkoutService";
+
+interface UseShippingMethodSelectorOptions {
+  checkoutId: string | null;
+  onSelect: (methodId: string) => void;
+  onRefresh: () => Promise<void>;
+}
+
+interface UseShippingMethodSelectorResult {
+  selectShippingMethod: (methodId: string) => Promise<void>;
+  selectShippingMethodWithApi: (methodId: string) => Promise<void>;
+}
+
+/**
+ * Hook to manage shipping method selection
+ * Encapsulates both UI state update and API communication
+ * Used for both manual selection (user click) and auto-selection (default method)
+ */
+export function useShippingMethodSelector(
+  options: UseShippingMethodSelectorOptions
+): UseShippingMethodSelectorResult {
+  const { checkoutId, onSelect, onRefresh } = options;
+
+  /**
+   * Updates UI state only (for initial/pre-selection)
+   */
+  const selectShippingMethod = useCallback(
+    async (methodId: string) => {
+      onSelect(methodId);
+    },
+    [onSelect]
+  );
+
+  /**
+   * Updates UI state AND calls Saleor API
+   * Use this when user manually selects OR when auto-selecting the default
+   */
+  const selectShippingMethodWithApi = useCallback(
+    async (methodId: string) => {
+      if (!checkoutId) {
+        console.warn("[selectShippingMethodWithApi] No checkoutId provided");
+        return;
+      }
+
+      // Update UI immediately for responsiveness
+      onSelect(methodId);
+
+      // Call API through CheckoutService
+      const checkoutService = createCheckoutService(checkoutId);
+      const result = await checkoutService.updateShippingMethod(methodId);
+
+      if (result.success) {
+        // Refresh checkout to get updated totals including shipping
+        await onRefresh();
+      } else {
+        console.error(
+          "[selectShippingMethodWithApi] Failed to update shipping method:",
+          result.error
+        );
+        // Could add error handling/rollback here
+      }
+    },
+    [checkoutId, onSelect, onRefresh]
+  );
+
+  return {
+    selectShippingMethod,
+    selectShippingMethodWithApi,
+  };
+}