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/[locale]/layout.tsx

@@ -1,12 +1,12 @@
 import { Metadata } from "next";
 import { NextIntlClientProvider } from "next-intl";
 import { getMessages, setRequestLocale } from "next-intl/server";
-import { routing } from "@/i18n/routing";
+import { SUPPORTED_LOCALES } from "@/lib/i18n/locales";
 
 const baseUrl = process.env.NEXT_PUBLIC_SITE_URL || "https://dev.manoonoils.com";
 
 export function generateStaticParams() {
-  return routing.locales.map((locale) => ({ locale }));
+  return SUPPORTED_LOCALES.map((locale) => ({ locale }));
 }
 
 export async function generateMetadata({
@@ -18,7 +18,7 @@ export async function generateMetadata({
   const localePrefix = locale === "sr" ? "" : `/${locale}`;
 
   const languages: Record<string, string> = {};
-  for (const loc of routing.locales) {
+  for (const loc of SUPPORTED_LOCALES) {
     const prefix = loc === "sr" ? "" : `/${loc}`;
     languages[loc] = `${baseUrl}${prefix}`;
   }