Notifications: Check if current device is correctly registered with backend and firebase

[sleidig]

Implement a method to NotificationService that checks if the current device is correctly registered for PushNotification with all required servers (firebase, aam-backend):

async checkDeviceRegistered(): Promise<boolean>

We need this to reliably display the push notification state/toggle to users and help them re-register the device if necessary.

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Before implementing the actual code, add unit tests for the following scenarios (by mocking different http/Notification.permission responses):

• browser push permission not granted --> expect false
• aam-backend does not have device registered --> expect false
• firebase does not have device registered --> expect false
• all of the above are correct --> expect true