fix(desktop): fall back to the engine's most recent session when the boot-restored chat is dead

[Vencounsel]

What does this PR do?

On cold start the desktop app navigates straight to the localStorage-pinned hermes.desktop.lastSessionId without validating it against the backend. When that pin is stale (the session was deleted or rotated while the app was closed), the boot strands on the resume-error screen for a chat the user never chose; the engine's actual most recent session is never considered.

This PR keeps the optimistic restore — the happy path still costs no round trip and no new-chat flash — and adds a fallback using the existing resume-exhausted latch as the staleness verdict: the latch only arms while the gateway is open, after the bounded resume retries gave up (use-route-resume.ts), so it cleanly distinguishes a dead pin from a down backend. No extra validation RPC, and no false positives from checking membership in the paginated session list.

When the latch fires for the boot-restored session while it's still the routed one, the controller replace-navigates to the engine's most recent session — preferring an is_active session (a runtime is live on it right now), then newest last_active, skipping archived rows and the dead session's own lineage — or to the new-chat route when nothing qualifies. The dead pin is still cleared, as before.

Deliberately narrow: sessions the user opened by hand keep the explicit error + Retry UI — the fallback fires only for the automatic boot restore, so it never yanks a session the user explicitly chose out from under them.

Related Issue

Fixes #60541

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• apps/desktop/src/app/desktop-controller.tsx: track which session id the cold-boot restore navigated to (bootRestoredSessionIdRef); on resume-exhaustion of exactly that session while routed to it, replace-navigate to pickMostRecentSessionId($sessions, deadId) or the new-chat route.
• apps/desktop/src/app/desktop-controller-utils.ts: new pure helper pickMostRecentSessionId() (active-first, then last_active; skips archived and the dead lineage — id matching mirrors sessionMatchesStoredId()).
• apps/desktop/src/app/desktop-controller-utils.test.ts: six cases — empty list, newest-by-last_active, active-over-newer-inactive, archived skipped, exclusion by id and lineage root, all-excluded → null.

How to Test

1. cd apps/desktop && npx vitest run --environment jsdom src/app/desktop-controller-utils.test.ts — 10 pass (4 existing + 6 new; the new ones fail on current main).
2. Manual reproduction: open a chat in the desktop app, quit, delete that session from the CLI (hermes sessions tooling) or another surface, relaunch.
   • Current main: boot lands on the resume-error screen for the deleted session.
   • This branch: after the bounded retries exhaust, the window jumps to the engine's most recent session (or new-chat when none), and the stale pin is replaced by the new session id.
3. Deliberate-open path unchanged: open an old dead session from the sidebar → error + Retry UI still shown, no auto-navigation.
4. npm run typecheck and npx eslint on the touched files pass.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(desktop):)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix
• I've run pytest tests/ -q: no Python changes in this diff; full-suite comparison against a pristine main checkout on the same machine shows no new failures (environment-dependent failures reproduce identically on base)
• I've added tests for my changes
• I've tested on my platform: macOS 26 (Darwin 25.5), Node 22.23.0
• Full desktop unit suite (vitest run --environment jsdom): no new failures vs. a pristine checkout of the same base commit (24 pre-existing environment-dependent failures on both, identical set)

Documentation & Housekeeping

• Comments updated in the touched effects — or N/A
• cli-config.yaml.example — N/A
• CONTRIBUTING.md / AGENTS.md — N/A
• Cross-platform impact: renderer-only
• Tool descriptions/schemas — N/A