[FEATURE] Desktop app: 'folder missing' section + bulk re-link for sessions whose project folder no longer exists

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet (rename-related prior art #55831/#52494 covers migration, not the stale-sidebar/orphan presentation below; both closed not-planned)
• [x] This is a single feature request (not multiple features)

Problem Statement

In the Claude Code desktop app, when a project folder no longer exists on disk (deleted, or renamed away), its sessions remain grouped in the sidebar under the dead path as if it were a live project:

• The sidebar shows a project group for a folder that doesn't exist — indistinguishable from real projects.
• Clicking a session inside it fails with "folder is no longer available", with no hint about what to do next.
• The transcripts still exist on disk (~/.claude/projects/<encoded-path>/*.jsonl), so the history isn't lost — but the UI gives no way to see that state or recover them except a per-session "change folder" action that forks each one individually.

Proposed Solution

When the desktop app detects that a project group's directory no longer exists:

• Move that group into a visually distinct "Unfiled / folder missing" section of the sidebar (instead of presenting it as a normal project), so history is preserved and clearly flagged rather than silently broken.
• On the group, offer bulk actions: "Re-link to folder…" (point all sessions at an existing directory at once) and "Archive".
• Opening a session from this state could prompt for a working directory instead of erroring.

Alternative Solutions

• Today: per-session "change folder", which forks one session at a time — workable for 2 sessions, painful for 20.
• Manually shuffling JSONL files under ~/.claude/projects/ — undocumented and fragile.

Priority

Medium - Would be very helpful

Feature Category

Other

Use Case Example

1. I renamed a repo folder; the old path's sessions stayed behind (see companion rename-migration request).
2. My sidebar now permanently shows the dead myproject-old project alongside real ones.
3. With this feature, those sessions would sit in an "Unfiled (folder missing)" section with a one-click "re-link all to ~/repos/myproject" — nothing lost, nothing phantom.

Additional Context

• Environment: Claude Code desktop app 2.1.214, macOS.
• Screenshot of the sidebar showing the phantom project group attached in a comment below.
• Companion request (separate per the single-feature rule): sessions should follow a renamed project folder automatically.