refactor(time): one timestamp formatter honouring the clock/date settings (#139)

Audit of every rendered time found five families of ad-hoc formatting:
the shared Time component + copies of its today/yesterday branch
(forwarded header, thread summary, read receipts, device tile, moderation
alerts, edit history), locale-default toLocale*String calls that ignored
the user's 12/24 h and date-format settings (scheduled tray, reminders,
schedule preview, notification snooze, bookmarks, threads list, search
cache line, room insights, media gallery), a hard-coded en-US date in the
activity log, and three relative-age variants.

utils/formatTimestamp.ts now holds the rules — today → time; yesterday /
tomorrow → day word + time; last 6 days → weekday + time; older → date +
time in dateFormatString — plus autoDate / time / date / dateTime styles,
formatDayDivider (full weekday), formatShortAge (room list) and
formatRelativeAge (list rows). useTimestampFormatter binds them to the
settings. 11 unit tests with an injected 'now'.

Visible changes are limited to consistency: 12 h times keep the existing
zero-padded hh:mm A; the a11y label and Created-by line use the user's
date format instead of a fixed long month.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/room/thread/ThreadsListPanel.tsx

@@ -27,6 +27,8 @@ import {
   isThreadSort,
 } from '../../../utils/threadList';
 import { useRoomThreads } from '../../../hooks/useRoomThreads';
+import { useTimestampFormatter } from '../../../hooks/useTimestampFormatter';
+import { formatRelativeAge } from '../../../utils/formatTimestamp';
 
 // Persisted across panel opens (the panel unmounts on close). getOnInit reads
 // localStorage synchronously so the chosen filter/sort apply on first render.
@@ -55,19 +57,6 @@ const SORT_OPTIONS: { value: ThreadSort; label: string }[] = [
 
 const MAX_PARTICIPANTS = 5;
 
-function formatTimeAgo(ts: number): string {
-  const diff = Date.now() - ts;
-  const minutes = Math.floor(diff / 60_000);
-  if (minutes < 1) return 'just now';
-  if (minutes < 60) return `${minutes}m ago`;
-  const hours = Math.floor(minutes / 60);
-  if (hours < 24) return `${hours}h ago`;
-  const days = Math.floor(hours / 24);
-  if (days === 1) return 'yesterday';
-  if (days < 7) return `${days}d ago`;
-  return new Date(ts).toLocaleDateString();
-}
-
 // Segmented button, mirroring the Bookmarks panel sort control for consistency.
 function SegButton({
   label,
@@ -115,6 +104,7 @@ type ThreadRowProps = {
   onOpen: (threadId: string) => void;
 };
 function ThreadRow({ room, thread, unread, highlight, participants, onOpen }: ThreadRowProps) {
+  const { prefs } = useTimestampFormatter();
   const rootEvent = thread.rootEvent;
   const rootSender = rootEvent?.getSender() ?? '';
   const { name: rootName, avatarUrl } = useMemberAvatar(room, rootSender);
@@ -126,7 +116,7 @@ function ThreadRow({ room, thread, unread, highlight, participants, onOpen }: Th
   const extra = participants.length - MAX_PARTICIPANTS;
   const replyLabel = `${count} ${count === 1 ? 'reply' : 'replies'}`;
   const ariaLabel = `Open thread by ${rootName}${unread > 0 ? ', unread' : ''}, ${replyLabel}${
-    typeof lastTs === 'number' ? `, last reply ${formatTimeAgo(lastTs)}` : ''
+    typeof lastTs === 'number' ? `, last reply ${formatRelativeAge(lastTs, prefs)}` : ''
   }`;
 
   return (
@@ -175,7 +165,7 @@ function ThreadRow({ room, thread, unread, highlight, participants, onOpen }: Th
         <Icon size="50" src={Icons.Thread} />
         <Text size="T200" priority="300" truncate style={{ flexGrow: 1 }}>
           {count} {count === 1 ? 'reply' : 'replies'}
-          {typeof lastTs === 'number' ? ` · ${formatTimeAgo(lastTs)}` : ''}
+          {typeof lastTs === 'number' ? ` · ${formatRelativeAge(lastTs, prefs)}` : ''}
         </Text>
         <Box shrink="No" alignItems="Center">
           {participants.slice(0, MAX_PARTICIPANTS).map((userId) => (