feat: toggle global DND when receiving a notification during scheduled DND

[nathanaelhoun]

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Description of Change

Scheduled DND now mute all notifications and sounds from all services, instead of "just" disabling the notification sent to the OS.

I also removed the periodic check of _systemDND on OSes where it's doesn't work ; it is only implemented on Mac at the moment.

Motivation and Context

I really like the Schedule DND feature, but as currently, it doesn't remove the sound of notifications of services like Mattermost or WhatsApp.
With this change, schedule DND works as I expected : it toggles the global switch during the scheduled times, instead of silently (because it's not shown in the UI) hiding the OS notification.

Other implementation

I would have preferred to activate and deactivate the global switch at the scheduled times, instead of checking it for each notification, but I didn't have the time to figure out how to schedule code in Electron.
This other implementation would allow users to manually deactivate DND in a scheduled period, or manually activate it while not in a scheduled period.

As this is a real problem, I will let this PR as draft until I figure out how to implement this correctly.

Screenshots

Checklist

• My pull request is properly named
• The changes respect the code style of the project (pnpm prepare-code)
• pnpm test passes
• I tested/previewed my changes locally

Release Notes

Scheduled DND now toggles the global "Disable notifications & audio" switch, which allows to mute all notifications sounds