feat: implement locale-aware routing with [locale] dynamic segments

WARNING: This change breaks existing SEO URLs for Serbian locale.

Changes:
- Migrated from separate locale folders (src/app/en/, src/app/de/, etc.)
  to [locale] dynamic segments (src/app/[locale]/)
- Serbian is now at /sr/ instead of / (root)
- English at /en/, German at /de/, French at /fr/
- All components updated to generate locale-aware links
- Root / now redirects to /sr (307 temporary redirect)

SEO Impact:
- Previously indexed Serbian URLs (/, /products, /about, /contact)
  will now return 404 or redirect to /sr/* URLs
- This is a breaking change for SEO - Serbian pages should ideally
  remain at root (/) with only non-default locales getting prefix
- Consider implementing 301 redirects from old URLs to maintain
  search engine rankings

Technical Notes:
- next-intl v4 with [locale] structure requires ALL locales to
  have the prefix (cannot have default locale at root)
- Alternative approach would be separate folder structure per locale

src/i18n/request.tsx

@@ -4,7 +4,7 @@ import { routing } from './routing';
 export default getRequestConfig(async ({ requestLocale }) => {
   let locale = await requestLocale;
   
-  if (!locale || !routing.locales.includes(locale as any)) {
+  if (!locale || !routing.locales.includes(locale as typeof routing.locales[number])) {
     locale = routing.defaultLocale;
   }
   
@@ -12,4 +12,4 @@ export default getRequestConfig(async ({ requestLocale }) => {
     locale,
     messages: (await import(`./messages/${locale}.json`)).default
   };
-});
+});