"Ungrouped" section should not render when there are no ungrouped sessions

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Title: "Ungrouped" section should not render when there are no ungrouped sessions

Environment

• Platform: Claude Code desktop app (macOS)
• App version: 1.24012.9
• macOS version: 26.5.2

Description

Once every session has been assigned to a group, the "Ungrouped" section holds nothing but still renders in the sidebar.

Unlike a user-created group, "Ungrouped" is a system fallback bucket. There is no user state to preserve when it is empty, so it should not be shown at all.

Steps to reproduce

1. Create one or more custom sidebar groups.
2. Move every session into a group so that none remain ungrouped.
3. Look at the bottom of the sidebar.

<img width="556" height="250" alt="Image" src="https://github.com/user-attachments/assets/ebadcab5-6322-4328-abf1-34fa95d83815" />

Expected behaviour

"Ungrouped" is hidden while it contains no sessions, and reappears as soon as a session becomes ungrouped.

Actual behaviour

"Ungrouped" stays visible with nothing under it.

Additional notes

Related but distinct: #77504 concerns the sort order within Ungrouped; this report is about the section rendering while empty.