NocoBase v2.0.0-alpha.9: Introduces support for custom variables

🎉 New Features

• [client] Add support for custom variables (#7585) by @zhangzhonghe

🚀 Improvements

• [AI employees] add minimize button for chat box In mobile layout (#7595) by @cgyrock

🐛 Bug Fixes

• [server] Preserve field.targetKey when copying reference fields (#7599) by @chenos

• [AI employees] fix chat box layout issue In mobile layout (#7591) by @cgyrock