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>

frontend/src/lib/messageGrouping.ts

@@ -1,7 +1,31 @@
 import { getUserAvatarUrl } from '../api/users'
 import { hashIndex } from './avatar'
+import { isSameLocalDay } from './formatMessageTime'
 import type { RoomMember } from '../types'
 
+// #73: Slack's own threshold for the same "still grouped, but it's been a
+// while" call -- past this gap a same-sender message starts a new group
+// (its own avatar/name/timestamp) even with nobody else posting in
+// between, so a message sent minutes later doesn't hide under a stale
+// timestamp from the start of the run.
+export const GROUP_BREAK_MS = 5 * 60 * 1000
+
+interface Groupable {
+  user_id: string
+  created_at: string
+}
+
+// Whether `msg` gets its own avatar/name/timestamp header, vs. continuing
+// `prev`'s group. A new calendar day always breaks the group: now that
+// timestamps can carry a date, a same-sender run that crosses midnight
+// (11:58 PM, then 12:01 AM) would otherwise leave the later message under a
+// header showing the wrong day.
+export function isGroupStart(prev: Groupable | undefined, msg: Groupable): boolean {
+  if (!prev || prev.user_id !== msg.user_id) return true
+  if (new Date(msg.created_at).getTime() - new Date(prev.created_at).getTime() > GROUP_BREAK_MS) return true
+  return !isSameLocalDay(prev.created_at, msg.created_at)
+}
+
 export function senderColorIndex(username: string, members: RoomMember[]): number {
   const idx = members.findIndex((m) => m.username === username)
   if (idx >= 0) return idx