fix(desktop): keep a just-finished session visible after switching away

[OutThisLife]

Summary

Fixes the report: "started a new session in the desktop app … after starting the session and then going to a different session, the session that I was in at the start just disappeared." (also a facet of #38989).

Root cause (a persist race, not a missing refresh): a brand-new session's first turn is written to the SessionDB a beat after the gateway emits message.complete. A refreshSessions() fired in that window gets a listSessions(min_messages=1) page that omits the new row. sessionsToKeep() already shields the active chat from this race — but a session you started and then navigated away from is, at the next refresh, neither working, pinned, nor active, so mergeSessionPage() evicts it. Nothing re-fetches afterward, so it stays gone until the app restarts.

This is distinct from #37908 (keep in-flight chats visible while working): here the turn has already finished, and the chat is no longer the active one.

Fix

Track sessions whose turn just settled — a real working → idle transition in setSessionWorking() — in a short, auto-expiring (30s) grace window, and add those ids to the merge keep-set in sessionsToKeep(). This bridges the persistence race for non-active chats.

• Only a genuine working → idle transition grants grace (updateSessionState re-asserts false on every idle tick — those must not pin an idle chat forever).
• Going busy again clears the grace entry.
• Entries auto-expire on read, so the set stays bounded with no timer.
• Cannot resurrect a deleted/archived session: mergeSessionPage() only revives rows still present in the in-memory list, which optimistic delete/archive already drop.

Changes

• store/session.ts — grace-window tracking (getRecentlySettledSessionIds()) wired into setSessionWorking().
• app/desktop-controller.tsx — include recently-settled ids in sessionsToKeep().
• store/session.test.ts — grace lifecycle coverage (window, idle re-assert no-op, re-busy clears).

Validation

	Before	After
Start chat → send → switch away before reply lands	new session vanishes from sidebar	stays visible; real row supersedes it on next refresh
Idle chat (no turn)	unaffected	unaffected (no grace granted)
Deleted/archived chat	stays gone	stays gone (not revived)

• vitest run src/store/session.test.ts — 13/13 passing
• tsc -b — clean
• eslint on changed files — 0 errors/warnings

Repro

Start a new chat, send a message, then click another session before the reply lands — the new session disappears from the sidebar.

[github-actions]

🔎 Lint report: fix/desktop-session-vanishes-after-switch vs origin/main

ruff

Total: 0 on HEAD, 0 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 0 pre-existing issues carried over.

ty (type checker)

Total: 10477 on HEAD, 10477 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 5495 pre-existing issues carried over.

Diagnostics are surfaced as warnings — this check never fails the build.