fix: user client failed to initialize in a deployed app

[fjakobs]

Without the fix we are getting this error in a deployed app with OBO enabled:

[{"error":{"json":{"message":"validate: more than one authorization method configured: oauth and pat. Config: host=adb-xxxxxx.azuredatabricks.net, token=***, client_id=yyyyyy, client_secret=***, warehouse_id=zzzzzz. Env: DATABRICKS_HOST, DATABRICKS_CLIENT_ID, DATABRICKS_CLIENT_SECRET, DATABRICKS_WAREHOUSE_ID","code":-32603,"data":{"code":"INTERNAL_SERVER_ERROR","httpStatus":500,"path":"executeSQL"}}}}]

[ditadi]

Not a blocker, but maybe it would be nice to log if the host doesn't exist.

 if (userToken && !host) {
    console.warn("[AppKit] User token present but DATABRICKS_HOST not set; user client unavailable");
  }