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}`);
+}