Describe the bug

Unread counts are not synchronised across clients, so when one has fully caught up with all of one's groups on client X and then immediately opens client Y on a different device, it's desirable to be able to mark all conversations as read. Unfortunately, this isn't possible, and one must instead mark each individual conversation as read.

If unread status cannot be synchronised across clients, a global 'Mark all as read' option would go some way towards easing the pain of using Session on multiple devices.