- Add middleware.ts for locale detection (URL path, cookie, Accept-Language)
- Update routing.ts to include en, de, fr locales
- Update layout.tsx with NextIntlClientProvider and dynamic lang attribute
- Create EN/DE/FR homepages, product listings, product details, about, and contact pages
- Serbian remains at root URL (/products, /about, /contact)
- English at /en/*, German at /de/*, French at /fr/*
