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/utils/datetimeInput.test.ts

@@ -55,16 +55,22 @@ test('formatFriendlyDateTime: uses Today/Tomorrow/date prefixes', () => {
   const tomorrow = new Date(2026, 0, 6, 9, 0).getTime();
   const nextWeek = new Date(2026, 0, 12, 9, 0).getTime();
 
-  assert.ok(formatFriendlyDateTime(laterToday, now).startsWith('Today at '));
-  assert.ok(formatFriendlyDateTime(tomorrow, now).startsWith('Tomorrow at '));
-  const other = formatFriendlyDateTime(nextWeek, now);
-  assert.ok(!other.startsWith('Today'));
-  assert.ok(!other.startsWith('Tomorrow'));
-  assert.ok(other.includes(' at '));
+  const prefs = { hour24Clock: true, dateFormatString: 'D MMM YYYY' };
+  assert.equal(formatFriendlyDateTime(laterToday, prefs, now), 'Today at 15:30');
+  assert.equal(formatFriendlyDateTime(tomorrow, prefs, now), 'Tomorrow at 09:00');
+  assert.equal(formatFriendlyDateTime(nextWeek, prefs, now), '12 Jan 2026 at 09:00');
+  assert.equal(
+    formatFriendlyDateTime(nextWeek, { hour24Clock: false, dateFormatString: 'MM/DD/YYYY' }, now),
+    '01/12/2026 at 09:00 AM',
+  );
 });
 
 test('formatFriendlyDateTime: Tomorrow rolls over month/year boundaries', () => {
   const nye = new Date(2026, 11, 31, 23, 0).getTime();
   const jan1 = new Date(2027, 0, 1, 9, 0).getTime();
-  assert.ok(formatFriendlyDateTime(jan1, nye).startsWith('Tomorrow at '));
+  assert.ok(
+    formatFriendlyDateTime(jan1, { hour24Clock: true, dateFormatString: '' }, nye).startsWith(
+      'Tomorrow at ',
+    ),
+  );
 });