Fix repeated charger notifications

What's fixed

"Charger connected" notifications firing over and over. With notifications enabled, the app could post a charger connect/disconnect pair about twice a second the whole time a charger stayed plugged in. A once-a-second background refresh was briefly clearing its internal list before rebuilding it, and the notification logic read that split-second gap as an unplug followed by a replug. The refresh now swaps in the new state in one step, and the notification check keys on the port a charger is on rather than a value that changes when the underlying service is recreated. Thanks @KelvinAyoub for the clear report (#227).

brew upgrade --cask whatcable or download from Releases.