* fix: use powerMonitor.on() only after app is ready
powerMonitor can't be used until the app is ready; however, on Linux,
powerMonitor.on() was called as soon as lib/browser/api/power-monitor.ts
was loaded.
This patch takes @vladimiry's suggestion of wrapping that in an
app.on('ready') handler to prevent powerMonitor.on() from being called
prematurely.
Fixes#21716