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/*

middleware.ts

@@ -0,0 +1,15 @@
+import createMiddleware from "next-intl/middleware";
+import { routing } from "./src/i18n/routing";
+
+export default createMiddleware({
+  ...routing,
+  localePrefix: "as-needed",
+});
+
+export const config = {
+  matcher: [
+    "/",
+    "/(sr|en|de|fr)/:path*",
+    "/((?!api|_next|_vercel|.*\\..*).*)",
+  ],
+};