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

middleware.ts

@@ -1,7 +1,8 @@
 import { NextResponse } from "next/server";
 import type { NextRequest } from "next/server";
+import { SUPPORTED_LOCALES, DEFAULT_LOCALE, LOCALE_COOKIE, getLocaleFromPath } from "@/lib/i18n/locales";
 
-const LOCALE_COOKIE = "NEXT_LOCALE";
+const OLD_SERBIAN_PATHS = ["products", "about", "contact", "checkout"];
 
 export default function middleware(request: NextRequest) {
   const pathname = request.nextUrl.pathname;
@@ -9,9 +10,9 @@ export default function middleware(request: NextRequest) {
   const acceptLanguage = request.headers.get("accept-language") || "";
 
   if (pathname === "/" || pathname === "") {
-    let locale = "sr";
+    let locale = DEFAULT_LOCALE;
 
-    if (cookieLocale && ["sr", "en", "de", "fr"].includes(cookieLocale)) {
+    if (cookieLocale && SUPPORTED_LOCALES.includes(cookieLocale as typeof SUPPORTED_LOCALES[number])) {
       locale = cookieLocale;
     } else if (acceptLanguage.includes("en")) {
       locale = "en";
@@ -22,15 +23,14 @@ export default function middleware(request: NextRequest) {
     return NextResponse.redirect(url, 301);
   }
 
-  const oldSerbianPaths = ["products", "about", "contact", "checkout"];
-  const isOldSerbianPath = oldSerbianPaths.some(
+  const isOldSerbianPath = OLD_SERBIAN_PATHS.some(
     (path) => pathname === `/${path}` || pathname.startsWith(`/${path}/`)
   );
 
   if (isOldSerbianPath) {
-    let locale = "sr";
+    let locale = DEFAULT_LOCALE;
 
-    if (cookieLocale && ["sr", "en", "de", "fr"].includes(cookieLocale)) {
+    if (cookieLocale && SUPPORTED_LOCALES.includes(cookieLocale as typeof SUPPORTED_LOCALES[number])) {
       locale = cookieLocale;
     } else if (acceptLanguage.includes("en")) {
       locale = "en";