feat: support cross-origin WebSocket proxy for remote gateway

- Add nginx /gwproxy/ reverse proxy with correct Origin header
- Auto-detect cross-origin gateway URL in GatewayClient.connect()
- Detect path-based proxy deployment in LoginScreen getInitialUrl()
- Change nginx/Docker to listen on port 3000, use host network mode

Dockerfile

@@ -10,7 +10,7 @@ RUN npm run build
 FROM nginx:alpine
 COPY --from=build /app/dist /usr/share/nginx/html
 COPY nginx.conf /etc/nginx/conf.d/default.conf
-EXPOSE 80
+EXPOSE 3000
 HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
-  CMD wget -qO /dev/null http://localhost:80/ || exit 1
+  CMD wget -qO /dev/null http://localhost:3000/ || exit 1
 CMD ["nginx", "-g", "daemon off;"]