refactor: centralize locale constants to prevent breaking changes

Created src/lib/i18n/locales.ts as single source of truth for:
- SUPPORTED_LOCALES array
- LOCALE_COOKIE name
- DEFAULT_LOCALE
- LOCALE_CONFIG (labels, flags, Saleor locale mapping)
- Helper functions (isValidLocale, getSaleorLocale, getLocaleFromPath)

Updated all files to use centralized constants:
- middleware.ts
- Header.tsx
- ProductCard.tsx
- sitemap.ts
- root layout and locale layout
- routing.ts

Benefits:
- Adding new locale only requires updating ONE file (locales.ts)
- No more hardcoded locale lists scattered across codebase
- Cookie name defined in one place
- Type-safe locale validation

src/app/layout.tsx

@@ -1,6 +1,7 @@
 import "./globals.css";
 import type { Metadata, Viewport } from "next";
 import ErrorBoundary from "@/components/providers/ErrorBoundary";
+import { SUPPORTED_LOCALES } from "@/lib/i18n/locales";
 
 const baseUrl = process.env.NEXT_PUBLIC_SITE_URL || "https://dev.manoonoils.com";
 
@@ -13,12 +14,9 @@ export const metadata: Metadata = {
   robots: "index, follow",
   alternates: {
     canonical: baseUrl,
-    languages: {
-      sr: baseUrl,
-      en: `${baseUrl}/en`,
-      de: `${baseUrl}/de`,
-      fr: `${baseUrl}/fr`,
-    },
+    languages: Object.fromEntries(
+      SUPPORTED_LOCALES.map((locale) => [locale, locale === "sr" ? baseUrl : `${baseUrl}/${locale}`])
+    ),
   },
   openGraph: {
     title: "ManoonOils - Premium Natural Oils for Hair & Skin",
@@ -48,4 +46,4 @@ export default async function RootLayout({
       </body>
     </html>
   );
-}
+}