fix: explorer.toml 恢复 REST API cluster，保留 cluster_native 辅助

deploy/config/explorer.toml

 # This configuration is also the target for data migration tasks.
 #
 # Default is "http://localhost:6041" - the default endpoint for REST API.
-# Note: taos-explorer now uses native connection instead of REST API.
+# Note: taos-explorer requires REST API for registration/login.
+# cluster_native is optional performance boost for data migration only.
 #
-# cluster = "http://localhost:6041"
+cluster = "http://172.21.185.173:6041"
 
 # native endpoint to connect to the cluster.
 # Default is disabled. To enable it, set it to the native API URL like "taos://localhost:6030" and uncomment it.