feat: add 301 redirects for old Serbian URLs to preserve SEO

Redirect old Serbian URLs (without /sr/ prefix) to new /sr/ URLs:
- / → /sr (301)
- /products → /sr/products (301)
- /about → /sr/about (301)
- /contact → /sr/contact (301)
- /checkout → /sr/checkout (301)

English URLs (/en/*) remain unchanged. This preserves SEO value
as Google treats 301 as permanent redirect passing ~90-99% PageRank.

middleware.ts

@@ -1,10 +1,42 @@
 import createMiddleware from "next-intl/middleware";
+import { NextResponse } from "next/server";
+import type { NextRequest } from "next/server";
 import { routing } from "./src/i18n/routing";
 
-export default createMiddleware({
+const oldSerbianPaths = ["", "products", "about", "contact", "checkout"];
+
+export default function middleware(request: NextRequest) {
+  const pathname = request.nextUrl.pathname;
+
+  const isOldSerbianPath = oldSerbianPaths.some((path) => {
+    if (path === "") {
+      return pathname === "/";
+    }
+    return pathname === `/${path}` || pathname.startsWith(`/${path}/`);
+  });
+
+  const hasLocalePrefix = routing.locales.some(
+    (locale) => pathname === `/${locale}` || pathname.startsWith(`/${locale}/`)
+  );
+
+  if (isOldSerbianPath && !hasLocalePrefix) {
+    const newPathname = pathname === "/"
+      ? "/sr"
+      : `/sr${pathname}`;
+    
+    const url = request.nextUrl.clone();
+    url.pathname = newPathname;
+    
+    return NextResponse.redirect(url, 301);
+  }
+
+  const intlMiddleware = createMiddleware({
     ...routing,
   });
 
+  return intlMiddleware(request);
+}
+
 export const config = {
   matcher: [
     "/",