We need a way to mark an entire session tree "as read" so we can clear the ma...

We need a way to mark an entire session tree "as read" so we can clear the markers. Often those sub sessions are just workers doing stuff and don't need to be read. In extreme cases, there can be dozens of them and having to touch each one just to clear the marker is tedious.

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| Field | Value |
| --- | --- |
| App version | 1.1.10 |
| OS | Windows 10.0.26200 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Week 6 |

从 /chat session-tree UI 开始，跟踪现有操作如何清除单个会话的未读标记。添加一个操作，将选中的会话及其所有后代标记为已读，然后验证树中的每个标记都已清除，同时不改变无关会话。

由索引模型根据 Issue 内容生成。