Support line-level review progress within a PR diff

Request

Please support line-level review progress within a PR diff.

Reasoning

For large changed files, file-level viewed/unviewed is too coarse. I often review a file in multiple passes and want a way to distinguish the lines I already reviewed from the lines I still need to look at.

Possible behavior

• mark selected lines or ranges as reviewed
• visually de-emphasize already-reviewed lines
• persist the state locally for the current PR / head SHA

A local-only feature would already be useful.

没有指定文件或测试。首先定位 PR diff UI 中现有的文件级已查看/未查看状态及其持久化路径。在实现可以视为完成之前，该 issue 需要确定范围选择、已审阅行显示，以及将存储状态限定到一个 PR 和一个 head SHA 的设计。