Adapt unread mail dashboard widget for mobile clients

[ChristophWurst]

Is your feature request related to a problem? Please describe.

With https://github.com/nextcloud/server/pull/33658 mobile clients can access app widget data.

Describe the solution you'd like

Adapt the Unread mail widget to https://github.com/nextcloud/server/pull/33658 once it's done.

Describe alternatives you've considered

No response

Additional context

No response