fix(desktop): harden smoke result persistence

[Whiteks1]

Summary

• persist structured smoke results even when Electron exits before the normal callback path completes
• harden the smoke harness so missing result.json becomes a clear structured failure instead of a raw ENOENT
• record Desktop/UI continuity for issue desktop(ui): harden desktop-smoke result persistence in CI #346

Issue

• Closes desktop(ui): harden desktop-smoke result persistence in CI #346

Validation

• node --check desktop/main.js
• node --check desktop/scripts/smoke.js
• npm install --include=dev in the clean worktree
• npm run smoke:fallback
• npm run smoke:real-path

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