Fix API connectivity in Docker Space

Root cause: NEXT_PUBLIC_API_URL was defaulting to 'http://localhost:8000' which made
the browser try to reach localhost instead of using Next.js proxy.

Fixes:
- Remove default value for NEXT_PUBLIC_API_URL in next.config.js
- Only set NEXT_PUBLIC_API_URL if explicitly provided
- In Docker Space, frontend uses relative URLs (empty string)
- Next.js server proxies /api/* to localhost:8000 backend
- Add console.log debugging to trace API URL resolution
- Add comments explaining Docker Space proxy architecture

This ensures:
✅ Production: Browser -> /api/models -> Next.js proxy -> localhost:8000/api/models
✅ Dev: Browser -> http://localhost:8000/api/models (direct)

Dockerfile

@@ -64,6 +64,8 @@ COPY --chown=user:user --from=frontend-builder /build/next.config.js ./frontend/
 COPY --chown=user:user --from=frontend-builder /build/node_modules ./frontend/node_modules
 
 # Set environment variables for the application
+# BACKEND_HOST is used by Next.js server for proxying
+# Do NOT set NEXT_PUBLIC_API_URL - let frontend use relative URLs
 ENV BACKEND_HOST=http://localhost:8000 \
     PORT=7860
 

frontend/next.config.js

@@ -2,13 +2,18 @@
 const nextConfig = {
   output: 'standalone',
   reactStrictMode: true,
-  env: {
-    NEXT_PUBLIC_API_URL: process.env.NEXT_PUBLIC_API_URL || 'http://localhost:8000',
-  },
+  // Only set NEXT_PUBLIC_API_URL if explicitly provided
+  // In Docker Space, we want it undefined so frontend uses relative URLs (proxied by Next.js)
+  ...(process.env.NEXT_PUBLIC_API_URL && {
+    env: {
+      NEXT_PUBLIC_API_URL: process.env.NEXT_PUBLIC_API_URL,
+    },
+  }),
   async rewrites() {
     // In Docker Space, proxy /api/* requests to backend on port 8000
     // This allows frontend (7860) to communicate with backend (8000) internally
     const API_HOST = process.env.BACKEND_HOST || 'http://localhost:8000';
+    console.log(`[Next.js] Setting up API proxy: /api/* -> ${API_HOST}/api/*`);
     return [
       {
         source: '/api/:path*',

frontend/src/lib/api.ts

@@ -13,23 +13,27 @@ import type {
 // Use relative URLs in production (Next.js rewrites will proxy to backend)
 // In local dev, use localhost:8000 for direct backend access
 const getApiUrl = () => {
-  // If explicitly set via env var, use it
+  // If explicitly set via env var, use it (for development)
   if (process.env.NEXT_PUBLIC_API_URL) {
+    console.log('[API Client] Using explicit API URL:', process.env.NEXT_PUBLIC_API_URL);
     return process.env.NEXT_PUBLIC_API_URL;
   }
   
   // For server-side rendering, always use relative URLs
   if (typeof window === 'undefined') {
+    console.log('[API Client] SSR mode: using relative URLs');
     return '';
   }
   
-  // On localhost (dev mode), use direct backend URL
+  // On localhost (dev mode), use direct backend URL  
   const hostname = window.location.hostname;
   if (hostname === 'localhost' || hostname === '127.0.0.1') {
+    console.log('[API Client] Localhost dev mode: using http://localhost:8000');
     return 'http://localhost:8000';
   }
   
   // In production (HF Space), use relative URLs (Next.js proxies to backend)
+  console.log('[API Client] Production mode: using relative URLs (proxied by Next.js)');
   return '';
 };