[BUG] Archiving a project doesn't archive its child sessions; manual archive afterward fails, sessions still show under active filter

## Preflight Checklist
- [x] I have searched existing issues and this hasn't been reported yet (closest related: #94698, which covers a different scenario — project rows with no bulk-archive action, not this state-inconsistency bug)
- [x] This is a single bug report
- [x] I am using the latest version of Claude Desktop

## What's Wrong?

When archiving a project that contains both Cowork sessions and chats, the child sessions are not archived along with the project. This produces three compounding issues:

1. After archiving the project, its Cowork sessions and chats remain in an unarchived state.
2. Attempting to manually archive one of those sessions afterward fails with the error "Impossible d'archiver cette tâche. Veuillez réessayer." (see attached screenshot behavior — the action is rejected outright, not just slow).
3. Sessions belonging to the archived project continue to appear in the sidebar/session tree even when filtering for "active" sessions only — they are neither archived nor correctly excluded from the active filter.

## Expected Behavior

Archiving a project should either:
- Cascade-archive all sessions (chat and Cowork) that belong to it, or
- If sessions are intentionally left unarchived, they should still be excludable from the "active sessions" filter once their parent project is archived, and manual archiving of those sessions should succeed rather than error.

## Steps to Reproduce

1. Create a project containing at least one Cowork session and one chat.
2. Archive the project.
3. Observe: sessions inside are not archived.
4. Try to manually archive one of those sessions.
5. Observe: error "Impossible d'archiver cette tâche. Veuillez réessayer."
6. Filter the session list to show only active sessions.
7. Observe: sessions from the archived project still appear.

## Environment

- Claude Desktop app
- macOS

No files or tests are named. Reproduce the issue in Claude Desktop using a project with one Cowork session and one chat, then trace project archiving, session archiving, and the active-session filter; done means child sessions are consistently archived or excluded and manual archiving succeeds.

Written by the indexing model from the issue text.