fix: terminate Electron startup smoke reliably

[numnx]

Summary

• terminate only the installed Electron startup probe through the Node process boundary after its readiness marker is durably written
• avoid the deferred Electron/AppKit exit path observed on current macOS release-candidate runners
• preserve the production before-quit server-drain path unchanged
• document and test the smoke-only termination contract

Why

The macOS release-candidate app successfully started its packaged backend and renderer and wrote the expected readiness marker, but remained alive after app.exit(0). The native smoke harness therefore timed out despite a healthy installation. This makes the probe exit deterministic without weakening the startup validation.

Validation

• pnpm exec vitest run tests/backend/electron/startup-smoke.test.ts tests/backend/electron-builder-config.test.ts — 10 passed
• pnpm run typecheck — passed
• pnpm run test:backend — 6,149 passed, 12 skipped
• pnpm run build — passed
• git diff --check — passed

Risk / rollback

Risk is limited to the opt-in installed-app smoke environment. Production desktop shutdown behavior is unchanged. Roll back commit 2ad49c415 if a runner-specific regression appears.