refactor: make locale handling truly centralized and robust

- Added getPathWithoutLocale() and buildLocalePath() helpers to locales.ts
- Updated Header to use centralized helpers instead of hardcoded regex
- Updated middleware to use SUPPORTED_LOCALES in matcher config
- Updated LocaleProvider to use isValidLocale() instead of hardcoded array

To add a new language now, only update:
1. SUPPORTED_LOCALES in locales.ts
2. LOCALE_CONFIG entry with label, flag, saleorLocale
3. Add translation keys to all message files

All routing now uses centralized constants - no more hardcoded locale lists.

src/components/layout/Header.tsx

@@ -9,7 +9,7 @@ import { useTranslations } from "next-intl";
 import { useSaleorCheckoutStore } from "@/stores/saleorCheckoutStore";
 import { User, ShoppingBag, Menu, X, Globe } from "lucide-react";
 import CartDrawer from "@/components/cart/CartDrawer";
-import { SUPPORTED_LOCALES, LOCALE_COOKIE, LOCALE_CONFIG, isValidLocale } from "@/lib/i18n/locales";
+import { SUPPORTED_LOCALES, LOCALE_COOKIE, LOCALE_CONFIG, isValidLocale, getPathWithoutLocale, buildLocalePath } from "@/lib/i18n/locales";
 import type { Locale } from "@/lib/i18n/locales";
 
 interface HeaderProps {
@@ -48,8 +48,8 @@ export default function Header({ locale = "sr" }: HeaderProps) {
       return;
     }
     document.cookie = `${LOCALE_COOKIE}=${newLocale}; path=/; max-age=31536000`;
-    const pathWithoutLocale = pathname.replace(/^\/(sr|en|de|fr)/, "") || "/";
-    const newPath = `/${newLocale}${pathWithoutLocale}`;
+    const pathWithoutLocale = getPathWithoutLocale(pathname);
+    const newPath = buildLocalePath(newLocale as Locale, pathWithoutLocale);
     window.location.replace(newPath);
     setLangDropdownOpen(false);
   };