feat: add language switcher with cookie persistence and browser detection

Changes:
- Root page.tsx now detects browser language (Accept-Language header)
  and cookie preference to redirect to correct locale (/sr or /en)
- Middleware handles old Serbian URLs (/products, /about, etc.) with
  301 redirects to /sr/* while respecting locale cookie
- Header component now includes language switcher dropdown with
  flag icons (Serbian and English)
- Language selection sets NEXT_LOCALE cookie and persists preference

Behavior:
- User visits / → redirects to /sr or /en based on cookie/browser
- User selects English from dropdown → cookie set, redirects to /en/*
- User visits /products with en cookie → /en/products (301)
- User visits /products with sr/no cookie → /sr/products (301)

middleware.ts

@@ -1,28 +1,25 @@
-import createMiddleware from "next-intl/middleware";
 import { NextResponse } from "next/server";
 import type { NextRequest } from "next/server";
-import { routing } from "./src/i18n/routing";
+
+const LOCALE_COOKIE = "NEXT_LOCALE";
 
 export default function middleware(request: NextRequest) {
   const pathname = request.nextUrl.pathname;
-
-  const hasLocalePrefix = routing.locales.some(
-    (locale) => pathname === `/${locale}` || pathname.startsWith(`/${locale}/`)
-  );
-
-  if (hasLocalePrefix) {
-    const intlMiddleware = createMiddleware({
-      ...routing,
-    });
-    return intlMiddleware(request);
-  }
+  const cookieLocale = request.cookies.get(LOCALE_COOKIE)?.value;
+  const acceptLanguage = request.headers.get("accept-language") || "";
 
   if (pathname === "/" || pathname === "") {
-    const intlMiddleware = createMiddleware({
-      ...routing,
-      localeDetection: true,
-    });
-    return intlMiddleware(request);
+    let locale = "sr";
+
+    if (cookieLocale && ["sr", "en", "de", "fr"].includes(cookieLocale)) {
+      locale = cookieLocale;
+    } else if (acceptLanguage.includes("en")) {
+      locale = "en";
+    }
+
+    const url = request.nextUrl.clone();
+    url.pathname = `/${locale}`;
+    return NextResponse.redirect(url, 302);
   }
 
   const oldSerbianPaths = ["products", "about", "contact", "checkout"];
@@ -31,16 +28,20 @@ export default function middleware(request: NextRequest) {
   );
 
   if (isOldSerbianPath) {
-    const newPathname = `/sr${pathname}`;
+    let locale = "sr";
+
+    if (cookieLocale && ["sr", "en", "de", "fr"].includes(cookieLocale)) {
+      locale = cookieLocale;
+    } else if (acceptLanguage.includes("en")) {
+      locale = "en";
+    }
+
     const url = request.nextUrl.clone();
-    url.pathname = newPathname;
+    url.pathname = `/${locale}${pathname}`;
     return NextResponse.redirect(url, 301);
   }
 
-  const intlMiddleware = createMiddleware({
-    ...routing,
-  });
-  return intlMiddleware(request);
+  return NextResponse.next();
 }
 
 export const config = {