Add frontend Dockerfile and integrate into docker-compose

- Multi-stage Dockerfile (deps → build → runner) for optimized production image
- Enable Next.js standalone output for minimal container size
- Add .dockerignore to exclude dev artifacts from build context
- Add frontend service to docker-compose.yml with API dependency

https://claude.ai/code/session_01QU6gpDgMtX4b9k1UXTivEf

frontend/next.config.ts

@@ -1,7 +1,7 @@
 import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
-  /* config options here */
+  output: "standalone",
 };
 
 export default nextConfig;