fix: blur not exit command mode

[HellowVirgil]

Description

Please include a summary of the changes and the motivation behind them. Also, include any relevant context or links to related issues.

Type of Change

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• Bug fix
• New feature
• Documentation update
• Code refactoring
• Other (please describe):

Related Issues

Please list any issues related to this pull request (e.g., Fixes #123, Closes #456).

Testing

Please describe the tests that were performed to verify your changes. Include details about the testing framework, if applicable.

• Unit tests
• Integration tests
• Manual testing

Checklist

• My code follows the project’s coding style.
• I have updated the documentation as necessary.
• I have added tests to cover my changes.
• All new and existing tests pass.
• My changes require a change to the documentation.

Additional Notes

• Notebook 的快捷键是按上下文条件启用的： focus && active && commandMode。当 Notebook 虽然失焦但 focus 没被清掉时，这些快捷键仍然处于启用状态。
• 当前失焦处理在 libro-view.tsx ，其中当 blur 事件的 relatedTarget 为空时直接 return ，没有把 libroService.focus 置空。很多从 Notebook 切到其他文件的场景， relatedTarget 为 null （例如点击非可聚焦区域、程序化切换视图），导致 focus 卡在 true ，Notebook 快捷键继续响应，出现数字、A、X 等按键在非 Notebook 文件仍触发删除、Markdown 转换等行为。

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