Send event reminder push notifications if there are no connected clients

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Is your feature request related to a problem? Please describe.
Use case 1: As a user of Nextcloud with connected clients like DAVx5, the sync client and Thunderbird I do not need Nextcloud to send notifications for events with reminders. The native apps already notify me.

Use case 2: As a user of Nextcloud who only uses Nextcloud via the web interface, I would like see a push notification for events with reminders.

Describe the solution you'd like

Change the default value for push notifications dynamically:

1. If the user has not enabled/disabled push notifications yet
   • Send notifications if there are no connected devices (no app passwords)
   • Don't send notifications if there is at least one connected device (app passwords exist)
2. If the user has disabled push notifications, don't send them
3. If the user has enabled push notifications, send them

Describe alternatives you've considered

N/a

Additional context

https://github.com/nextcloud/server/pull/16970 made the feature opt-in

Start by reviewing the existing opt-in behavior introduced in PR #16970 and tracing push-notification preference handling alongside app-password and connected-device detection. Done means unconfigured users receive reminders only without connected devices, while explicit enablement or disablement continues to take precedence.