desktop(ui): harden desktop-smoke result persistence in CI

[Whiteks1]

Problem

desktop-smoke is failing in GitHub Actions before the smoke harness can read result.json.

Observed failure in PR #345:

• job: desktop-smoke
• error: ENOENT: no such file or directory, open '/tmp/quantlab-desktop-smoke-*/result.json'
• reproduced twice in the same PR, including a rerun

Why this blocks work

This blocks even planning-only Desktop/UI PRs because the CI failure happens before the harness can emit a structured smoke result.

Scope

Desktop/UI only.

Likely files:

• desktop/scripts/smoke.js
• desktop/main.js if result persistence depends on main-process exit timing

Done when

• CI no longer fails with missing result.json
• smoke emits a structured failure when Electron exits too early
• planning-only PRs are not blocked by an unrelated smoke harness crash