Manual updater hangs if network is unreachable or slow, cannot be canceled

[codebykat]

I'm on Amtrak wifi and noticed the update check hangs indefinitely (whether because the network is slower than molasses or because it's not able to reach the server, I can't tell) and it is also not cancellable.

I did a search of electron-updater issues and docs and there's apparently no way to add a timeout, but maybe we can wrap it in one? We should also allow the modal to be dismissed, whether this cancels the check or just backgrounds it so that it's possible to go back to your notes. Right now all you can do is close the app.