Show the date on message timestamps that aren't from today

A bare "4:04 PM" gave no way to tell today's message from one sent last
week. Today stays time-only; older messages gain just enough date to
place them: "Yesterday 4:04 PM", a weekday for the past week ("Mon
4:04 PM"), then "Oct 2, 4:04 PM" (with the year if it isn't this one).
Hovering a time shows the full date. All in the viewer's local time.

A new calendar day now always starts a new message group. Previously a
same-sender run that crossed midnight within the 5-minute window (11:58
PM then 12:01 AM) stayed under one header, which would have put the
later message under the wrong day's timestamp. The grouping rule moves
out of MessageList into messageGrouping.ts so it's unit-tested.

useLocalDay refreshes "today" at local midnight and when a hidden tab
becomes visible again, so a tab left open overnight doesn't keep showing
yesterday's messages as a bare time. .message-time no longer wraps, and
.message-author can shrink, so a long display name can't push the longer
label out of the row on a phone.

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

USER_GUIDE.md

@@ -68,6 +68,11 @@ new message, or if you message them again yourself.
 Type in the message box at the bottom of a room and press **Enter** to
 send. Use **Shift+Enter** to add a line break without sending.
 
+Each message shows when it was sent, in your local time. Messages from today
+show just the time; older ones add the day — "Yesterday 4:04 PM", a weekday
+like "Mon 4:04 PM" for the past week, then the date ("Oct 2, 4:04 PM", with
+the year if it's not this one). Hover a time to see the full date.
+
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