Improve chat channel unread marker handling

[notbakaneko]

Decouples the previous markAsRead from setting the unread marker position and uses the marker position when marking as read instead of assuming the last message.

There was an issue where when scrolling to the bottom of a channel and then scrolling back up and down again, if the scroll was not quite at the bottom of the channel when switching, it would not mark the channel as read.

(As an aside, this also means the client can just set the marker to some midway point instead of the end)

should fix #9003

[nanaya]

what's the exact repro steps for this?

[notbakaneko]

1. switch to channel with unread message
2. scroll to bottom
3. scroll up 1px
4. switch away -> channel still unread (should be considered read because got scrolled to bottom while window had focus)

cases where it shouldn't be considered read:

• window does not have focus
• did not scroll to bottom before switching away

[nanaya]

shouldn't this check if LastSent >= lastReadId as well? updateWithJson may update the value outside this function (and the same check when setting the value in there?)

[notbakaneko]

not sure the delay is even needed, thinking about it again...