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/lib/i18n/locales.ts

@@ -0,0 +1,26 @@
+export const SUPPORTED_LOCALES = ["sr", "en", "de", "fr"] as const;
+export type Locale = (typeof SUPPORTED_LOCALES)[number];
+
+export const DEFAULT_LOCALE: Locale = "sr";
+
+export const LOCALE_COOKIE = "NEXT_LOCALE";
+
+export const LOCALE_CONFIG: Record<Locale, { label: string; flag: string; saleorLocale: string }> = {
+  sr: { label: "Srpski", flag: "🇷🇸", saleorLocale: "SR" },
+  en: { label: "English", flag: "🇬🇧", saleorLocale: "EN" },
+  de: { label: "Deutsch", flag: "🇩🇪", saleorLocale: "EN" },
+  fr: { label: "Français", flag: "🇫🇷", saleorLocale: "EN" },
+};
+
+export function isValidLocale(locale: string): locale is Locale {
+  return SUPPORTED_LOCALES.includes(locale as Locale);
+}
+
+export function getSaleorLocale(locale: Locale): string {
+  return LOCALE_CONFIG[locale].saleorLocale;
+}
+
+export function getLocaleFromPath(pathname: string): string {
+  const match = pathname.match(/^\/(sr|en|de|fr)/);
+  return match ? match[1] : DEFAULT_LOCALE;
+}