Messaging · Messages: messages in runs, threads, the unread line and receipts

ChatMessage sets a message under its author's picture, name and clock
time. The next by the same author within five minutes, on the same day,
joins the run: only its words, the time in the gutter on hover.
UnreadDivider marks where unread messages start. MessageThread tells who
replied, how many and when the last came; with a handler it opens on a
press, Enter or Space. ThreadPanel holds the first message, a rule
counting the replies, the replies and a composer, its body scrolling.
ReadReceipt says sending, sent, delivered or read, and who saw it in a
group. MessageReactions and TypingIndicator point to collab and motion.

gpui recolors nothing on hover: text takes its color at layout. The
gutter time rests invisible and turns visible through a group hover.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tasks/ch21-30.md

@@ -330,13 +330,13 @@
 
 ## T28b Messaging · Messages
 
-- [ ] ChatMessage
-- [ ] MessageThread
-- [ ] ThreadPanel
-- [ ] UnreadDivider
-- [ ] MessageReactions → `collab::Reactions`
-- [ ] ReadReceipt
-- [ ] TypingIndicator → `motion::TypingIndicator`
+- [x] ChatMessage
+- [x] MessageThread
+- [x] ThreadPanel
+- [x] UnreadDivider
+- [x] MessageReactions → `collab::Reactions`
+- [x] ReadReceipt
+- [x] TypingIndicator → `motion::TypingIndicator`
 
 ## T28c Messaging · People