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/lib/i18n/locales.ts

@@ -21,6 +21,19 @@ export function getSaleorLocale(locale: Locale): string {
 }
 
 export function getLocaleFromPath(pathname: string): string {
-  const match = pathname.match(/^\/(sr|en|de|fr)/);
+  const pattern = SUPPORTED_LOCALES.join("|");
+  const match = pathname.match(new RegExp(`^\\/(${pattern})`));
   return match ? match[1] : DEFAULT_LOCALE;
 }
+
+export function getPathWithoutLocale(pathname: string): string {
+  const pattern = SUPPORTED_LOCALES.join("|");
+  return pathname.replace(new RegExp(`^\\/(${pattern})`), "") || "/";
+}
+
+export function buildLocalePath(locale: Locale, path: string): string {
+  if (locale === DEFAULT_LOCALE) {
+    return path === "/" ? "/" : path;
+  }
+  return `/${locale}${path === "/" ? "" : path}`;
+}