Add electron-updater manual install flow

[enaboapps]

Closes #151

Summary

• Add electron-updater and explicit GitHub publish metadata for updater config generation.
• Replace the custom GitHub release downloader with an electron-updater-backed manual check/download/install flow.
• Replace Show in folder with Install update after an update is downloaded.
• Keep automatic download and automatic install disabled.

Validation

• npm test
• npm run typecheck
• npm run build
• npm run package:win with Certum signing mode
• npm run package:win:verify-uiaccess
• signtool verify /pa /v dist\Switchify-PC-Setup-0.1.0-x64.exe
• verified dist\latest.yml and dist\win-unpacked\resources\app-update.yml are generated