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/i18n/routing.ts

@@ -1,7 +1,8 @@
 import { defineRouting } from "next-intl/routing";
+import { SUPPORTED_LOCALES, DEFAULT_LOCALE } from "@/lib/i18n/locales";
 
 export const routing = defineRouting({
-  locales: ["sr", "en", "de", "fr"],
-  defaultLocale: "sr",
+  locales: SUPPORTED_LOCALES,
+  defaultLocale: DEFAULT_LOCALE,
   localePrefix: "as-needed",
-});
+});