Show a dot on a channel a Bot has spoken in unseen (#259)

* Give a membership a memory of when its channel was last read

* Stamp the caller's membership read, and say so in the roster

* Let a member say they have read a channel

* Carry the read marker to the app and let it be stamped

* Draw a dot on a channel a Bot has spoken in unseen

* Mark a channel read the moment it is the one on screen

* Narrow the admin and settings rails to the width their labels earn

* Keep a fast clock elsewhere from turning mark-read into a storm

* Say in the changelog what the unread dot is and is not

* Stamp a read against the message clock, not just this one

* Assert the clamped stamp without reaching through an optional chain

* Regenerate the read-marker migration behind the queue and the frames

* Refuse to mark a deleted channel read, matching the pin