fix: use standalone nginx config without backend upstream in standalone mode

Nginx fails to start when the 'backend' upstream doesn't exist.
Standalone build now copies a minimal nginx config that only serves the SPA.

docker/nginx.standalone.conf

@@ -0,0 +1,11 @@
+server {
+    listen 80;
+    server_name _;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # SPA fallback — no backend proxy
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+}