Sidebar shows "session file could not be opened, link or placeholder" for active sessions after restart

## Description
The session history sidebar shows the error "Diese Sitzung konnte nicht geladen werden / Diese Sitzungsdatei wurde nicht geöffnet, da es sich offenbar um einen Link oder einen Platzhalter und nicht um eine normale Datei handelt." ("This session could not be loaded / This session file was not opened, because it appears to be a link or a placeholder rather than a normal file.") for sessions in the sidebar, including the currently active, live session.

## Steps to reproduce
1. Have an active conversation in the desktop app
2. Restart the app (fully closed and reopened)
3. The sidebar entry for the session (including the one just restored/active) shows the above error
4. Clicking "Retry" does not resolve it

## Expected behavior
Sessions should load normally in the sidebar.

## Actual behavior
Error dialog shown, persists even after a clean app restart.

## Additional context
- Verified on disk that the underlying `.jsonl` transcript file for the affected session is a completely normal, valid, regular file (not a symlink or reparse point), checked multiple times including immediately before and after the error appeared.
- Happens even while the session is actively being used (live, ongoing conversation), not just for old/stale sessions.
- Platform: Windows 10 Pro 10.0.19045, project directory on local disk, some other projects on a mapped network share.
- "Retry" button in the error dialog does not recover the session.

Reproduce on Windows by starting an active conversation in the desktop app, fully restarting it, and checking the session history sidebar. Inspect the session-loading path for sidebar entries and how it opens the underlying .jsonl transcript files. Done means normal regular transcript files load after restart, including the active session, and Retry no longer leaves the same error.

索引モデルが issue の本文から書いたものです。