[INFRA] Extend CI with Windows smoke (packaged `dir` or minimal Electron launch)

[krwg]

Summary

Current CI (ubuntu-latest) validates npm ci + npm run build (Vite). Add an optional Windows job that installs deps and runs electron-builder --dir or equivalent to catch Windows-only packaging regressions before tagging.

Acceptance criteria

• New workflow job windows (or matrix) on PR + main.
• Caches npm; runtime < ~15 min on cold start where possible.
• Document secrets requirement if code signing introduced later.

Out of scope (for first iteration)

Fully headless codec / WebRTC E2E.