Add Docker support (standalone Next.js output)

Multi-stage Dockerfile producing a minimal runtime image via
next.config.ts's output:"standalone". NEXT_PUBLIC_API_URL is taken as
a build arg since it gets baked into the client bundle at build time.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

next.config.ts

@@ -1,7 +1,8 @@
 import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
-  /* config options here */
+  // برای build خروجی مستقل (بدون نیاز به node_modules کامل) — لازم برای ایمیج Docker سبک
+  output: "standalone",
 };
 
 export default nextConfig;