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.

Dockerfile.frontend

@@ -16,7 +16,13 @@ FROM nginx:alpine
 
 COPY --from=builder /app/dist /usr/share/nginx/html
 
-# Nginx config: proxy /api and /ws to backend, serve SPA
+# Nginx config: use standalone config (no backend proxy) or full config
+ARG VITE_STANDALONE=false
 COPY docker/nginx.conf /etc/nginx/conf.d/default.conf
+COPY docker/nginx.standalone.conf /etc/nginx/conf.d/nginx.standalone.conf
+RUN if [ "$VITE_STANDALONE" = "true" ]; then \
+      cp /etc/nginx/conf.d/nginx.standalone.conf /etc/nginx/conf.d/default.conf; \
+    fi && \
+    rm /etc/nginx/conf.d/nginx.standalone.conf
 
 EXPOSE 80