Moving a session to an existing project name ungroups all sessions

Description

Moved a chat/session to a different project via the UI's "move to project" action. The UI prompted for a project name; I entered the name of a project that already existed (rather than a new one). After confirming, every session across every project reverted to "Ungrouped" in the sidebar -- not just the session I moved.

Steps to reproduce

1. Open the session list/sidebar with sessions already grouped under multiple existing projects.
2. Pick a session and choose "move to project" (or equivalent).
3. When prompted for the project name, enter the name of a project that already exists.
4. Confirm the move.

Expected behavior
The session moves into the existing project group; all other sessions keep their existing group assignments.

Actual behavior
All sessions across all projects fell back to "Ungrouped." Restarting the app did not restore the groupings.

Notes

• No apparent data loss -- session transcripts remain correctly filed under their original project folders on disk (e.g. under ~/.claude/projects/<project>/), so this looks like a bug in server-side/session grouping state rather than data loss.
• Suspected cause: the move-to-existing-project-name path likely hits a merge/dedup code path that isn't handled cleanly, and clears grouping globally instead of merging just the one session into the target group.

Environment

• Claude Code version: 2.1.229
• Platform: Windows 11