test: verify Electron app packages and launches

[icebear0828]

Goal

Write a test or CI job that verifies the Electron app can build and launch without crashing.

Acceptance criteria

• cd packages/electron && npm run build succeeds
• Electron binary can be packaged via electron-builder
• App launches and the main window opens (use Playwright or xvfb for headless)
• App exits cleanly

Context

• Electron code lives in packages/electron/
• Build script: packages/electron/electron/build.mjs
• Packaging: packages/electron/electron/prepare-pack.mjs + electron-builder
• Existing tests: packages/electron/__tests__/ (40 tests, but no launch smoke test)

[icebear0828]

Superseded by #134 — rewritten with explicit project constraints (npm only, master branch, no version bumps).