Refresh the mobile unread indicator on DOM update

Refresh the mobile unread indicator on DOM update

The unread indicator might become out of sync if the menu is closed, but an action causes the last remaining element(s) to be marked as read.

Using a `MutationObserver` removes the need of signaling such change directly by all actions, for example a “Mark All as Read” button.

See https://www.woltlab.com/community/thread/296063-alle-als-gelesen-markieren-entfernt-badge-nicht/