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.md

@@ -98,7 +98,7 @@ Per-component lines live in `tasks/`. Tags there:
 - [x] T27b Files · Explorer
 - [x] T27c Files · Find and look
 - [x] T27d Files · Transfers
-- [ ] T28a Messaging · Channels
+- [x] T28a Messaging · Channels
 - [ ] T28b Messaging · Messages
 - [ ] T28c Messaging · People
 - [ ] T28d Messaging · Calls
@@ -333,3 +333,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T27c | 2 | PASS | |
 | T27d | 1 | FAIL | a press on a finished download's Show button also opened the file through its row |
 | T27d | 2 | PASS | |
+| T28a | 1 | PASS | |