Sidebar session list repeatedly goes empty despite session data being intact

### Description
The desktop app's left sidebar session list intermittently goes empty — all sessions disappear from the panel — even though the underlying session data is confirmed intact.

### Steps observed
- Sidebar showed 20+ sessions, then went empty with no error shown.
- Recurred on two separate occasions: 2026-08-20 and 2026-08-21.

### Confirmed NOT data loss
Queried the session store directly both times via the session management API — all sessions (49+, going back to mid-June) were present, none archived, none deleted. The sidebar's fetch/render layer is failing silently while the backend data is fine.

### Troubleshooting already tried (did not fix it)
- Reload / hard refresh
- Full quit and relaunch (not just closing the window)
- Checking the workspace/folder filter
- Checking for a stray search term in the sidebar search box

### Environment
- Desktop app (Mac)

Start at the desktop app's sidebar session fetch/render layer and compare its behavior with the session management API, using the reported Mac environment and the intermittent empty-list behavior as the reproduction target. Done means the sidebar continues to show the intact sessions instead of silently becoming empty.

索引モデルが issue の本文から書いたものです。