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)

src/app/page.tsx

@@ -1,5 +1,18 @@
 import { redirect } from "next/navigation";
+import { cookies, headers } from "next/headers";
 
-export default function RootPage() {
-  redirect("/sr");
-}
+export default async function RootPage() {
+  const headersList = await headers();
+  const cookieStore = await cookies();
+  const acceptLanguage = headersList.get("accept-language") || "";
+  const cookieLocale = cookieStore.get("NEXT_LOCALE")?.value;
+
+  let locale = "sr";
+  if (cookieLocale && ["sr", "en", "de", "fr"].includes(cookieLocale)) {
+    locale = cookieLocale;
+  } else if (acceptLanguage.includes("en")) {
+    locale = "en";
+  }
+
+  redirect(`/${locale}`);
+}

src/components/layout/Header.tsx

@@ -1,27 +1,57 @@
 "use client";
 
-import { useState, useEffect } from "react";
+import { useState, useEffect, useRef } from "react";
 import Link from "next/link";
 import Image from "next/image";
+import { useRouter, usePathname } from "next/navigation";
 import { AnimatePresence, motion } from "framer-motion";
 import { useTranslations } from "next-intl";
 import { useSaleorCheckoutStore } from "@/stores/saleorCheckoutStore";
-import { User, ShoppingBag, Menu, X } from "lucide-react";
+import { User, ShoppingBag, Menu, X, Globe } from "lucide-react";
 import CartDrawer from "@/components/cart/CartDrawer";
 
+const LOCALES = [
+  { code: "sr", label: "Srpski", flag: "🇷🇸" },
+  { code: "en", label: "English", flag: "🇬🇧" },
+] as const;
+
 interface HeaderProps {
   locale?: string;
 }
 
 export default function Header({ locale = "sr" }: HeaderProps) {
   const t = useTranslations("Header");
+  const router = useRouter();
+  const pathname = usePathname();
+  const dropdownRef = useRef<HTMLDivElement>(null);
   const [mobileMenuOpen, setMobileMenuOpen] = useState(false);
   const [scrolled, setScrolled] = useState(false);
+  const [langDropdownOpen, setLangDropdownOpen] = useState(false);
   const { getLineCount, toggleCart, initCheckout } = useSaleorCheckoutStore();
 
   const itemCount = getLineCount();
   const localePath = `/${locale}`;
 
+  const currentLocale = LOCALES.find((l) => l.code === locale) || LOCALES[0];
+
+  useEffect(() => {
+    const handleClickOutside = (event: MouseEvent) => {
+      if (dropdownRef.current && !dropdownRef.current.contains(event.target as Node)) {
+        setLangDropdownOpen(false);
+      }
+    };
+    document.addEventListener("mousedown", handleClickOutside);
+    return () => document.removeEventListener("mousedown", handleClickOutside);
+  }, []);
+
+  const switchLocale = (newLocale: string) => {
+    document.cookie = `NEXT_LOCALE=${newLocale}; path=/; max-age=31536000`;
+    const currentPath = pathname.replace(`/${locale}`, "") || "/";
+    const newPath = newLocale === "sr" ? currentPath : `/${newLocale}${currentPath}`;
+    router.push(newPath);
+    setLangDropdownOpen(false);
+  };
+
   useEffect(() => {
     initCheckout();
   }, [initCheckout]);
@@ -94,6 +124,40 @@ export default function Header({ locale = "sr" }: HeaderProps) {
           </Link>
 
           <div className="flex items-center gap-1">
+            <div ref={dropdownRef} className="relative">
+              <button
+                className="p-2 hover:bg-black/5 rounded-full transition-colors flex items-center gap-1"
+                onClick={() => setLangDropdownOpen(!langDropdownOpen)}
+                aria-label="Select language"
+              >
+                <Globe className="w-5 h-5" strokeWidth={1.5} />
+                <span className="text-sm">{currentLocale.flag}</span>
+              </button>
+              <AnimatePresence>
+                {langDropdownOpen && (
+                  <motion.div
+                    initial={{ opacity: 0, y: -10 }}
+                    animate={{ opacity: 1, y: 0 }}
+                    exit={{ opacity: 0, y: -10 }}
+                    className="absolute right-0 top-full mt-1 bg-white border border-[#e5e5e5] shadow-lg rounded-md overflow-hidden z-50"
+                  >
+                    {LOCALES.map((loc) => (
+                      <button
+                        key={loc.code}
+                        onClick={() => switchLocale(loc.code)}
+                        className={`flex items-center gap-2 px-4 py-2 text-sm hover:bg-black/5 transition-colors w-full text-left ${
+                          loc.code === locale ? "bg-black/5 font-medium" : ""
+                        }`}
+                      >
+                        <span>{loc.flag}</span>
+                        <span>{loc.label}</span>
+                      </button>
+                    ))}
+                  </motion.div>
+                )}
+              </AnimatePresence>
+            </div>
+
             <button
               className="p-2 hover:bg-black/5 rounded-full transition-colors hidden sm:block"
               aria-label={t("account")}