This PR ensures threads created on an empty window (i.e., no
paths/worktrees associated with it) can still be displayed in the thread
archive view. They can't be unarchived, though, as we don't know where
we'd insert it, even if we had a project on the window. Lastly, I'm also
hiding the "current worktree"/"new git worktree" menu from the agent
panel if we don't have any paths as those options don't make sense in
this case.

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A