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 |

Start in the /chat session-tree UI and trace how the existing action clears a single session's unread marker. Add an action that marks the selected session and all descendants as read, then verify that every marker in the tree is cleared without changing unrelated sessions.

Written by the indexing model from the issue text.