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/components/layout/Header.tsx

@@ -55,14 +55,14 @@ export default function Header({ locale: propLocale = "sr" }: HeaderProps) {
     setLangDropdownOpen(false);
   };
 
-  // Set language code first, then initialize checkout
+  // Set language code - checkout initializes lazily when cart is opened
   useEffect(() => {
     if (locale) {
       setLanguageCode(locale);
-      // Initialize checkout after language code is set
-      initCheckout();
+      // Checkout will initialize lazily when user adds to cart or opens cart drawer
+      // This prevents blocking page render with unnecessary API calls
     }
-  }, [locale, setLanguageCode, initCheckout]);
+  }, [locale, setLanguageCode]);
 
   useEffect(() => {
     const handleScroll = () => {