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/providers/LocaleProvider.tsx

@@ -3,6 +3,7 @@
 import { NextIntlClientProvider } from "next-intl";
 import { getMessages } from "next-intl/server";
 import { notFound } from "next/navigation";
+import { SUPPORTED_LOCALES, isValidLocale } from "@/lib/i18n/locales";
 
 export default async function LocaleProvider({
   children,
@@ -11,8 +12,7 @@ export default async function LocaleProvider({
   children: React.ReactNode;
   locale: string;
 }) {
-  const locales = ["en", "sr"];
-  if (!locales.includes(locale)) notFound();
+  if (!isValidLocale(locale)) notFound();
 
   const messages = await getMessages();