code-signed builds and auto-update

https://electronjs.org/docs/api/auto-updater

* opt-in
* display feedback when update is available/downloading
* "update downloaded. restart now?" popup

now that GitHub Actions builds releases, this would be particularly cool since you'd be able to deploy updates to people's apps by pushing a git tag.

Start with the Electron auto-updater documentation linked in the issue and review how GitHub Actions currently builds releases. Define the opt-in flow, update-available and download feedback, code-signed distribution, and the restart prompt; done means releases can deliver updates to installed apps with those interactions working.

Written by the indexing model from the issue text.