fix(frontend): use configurable backend URL in Next.js rewrite

The rewrite was using 'http://backend:8000' which only resolves inside
Docker network. When running Next.js locally (npm run dev), the hostname
'backend' doesn't exist, causing ENOTFOUND errors.

Now uses NEXT_PUBLIC_API_BASE_URL env var with fallback to localhost:8000
for local development. In Docker, set NEXT_PUBLIC_API_BASE_URL=http://backend:8000.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/next.config.ts

@@ -74,12 +74,14 @@ const nextConfig: NextConfig = {
     ];
   },
 
-  // Ensure we can connect to the backend in Docker
+  // Proxy API requests to backend
+  // Use NEXT_PUBLIC_API_BASE_URL for the destination (defaults to localhost for local dev)
   async rewrites() {
+    const backendUrl = process.env.NEXT_PUBLIC_API_BASE_URL || 'http://localhost:8000';
     return [
       {
         source: '/api/:path*',
-        destination: 'http://backend:8000/api/:path*',
+        destination: `${backendUrl}/api/:path*`,
       },
     ];
   },