feat: phase 1 - i18n core infrastructure with EN/DE/FR locales

- 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/*

src/i18n/routing.ts

@@ -1,7 +1,7 @@
-import { defineRouting } from 'next-intl/routing';
+import { defineRouting } from "next-intl/routing";
 
 export const routing = defineRouting({
-  locales: ['sr', 'en'],
-  defaultLocale: 'sr',
-  localePrefix: 'as-needed'
+  locales: ["sr", "en", "de", "fr"],
+  defaultLocale: "sr",
+  localePrefix: "as-needed",
 });