perf(mobile): cache calendar conversions within timeline builds

Signed-off-by: Tom Brow <tomb@block.xyz>

mobile/lib/features/channels/channel_detail_page/message_list.dart

@@ -56,6 +56,7 @@ class _MessageList extends HookConsumerWidget {
       localMessageSendAnimationProvider(channelId),
     );
     final displayEntries = groupMembershipTimelineEntries(entries);
+    final calendarDays = TimelineCalendarDays();
     final itemScrollController = useMemoized(ItemScrollController.new);
     final itemPositionsListener = useMemoized(ItemPositionsListener.create);
     final stickyDayTimestamp = useValueNotifier<int?>(null);
@@ -121,7 +122,7 @@ class _MessageList extends HookConsumerWidget {
           : null;
       final startsDay =
           previousMessage == null ||
-          !isSameDay(previousMessage.createdAt, message.createdAt);
+          !calendarDays.isSameDay(previousMessage.createdAt, message.createdAt);
       final reversedIndex = displayEntries.length - 1 - chronologicalIndex;
       if (startsDay) {
         currentDayTimestamp = message.createdAt;
@@ -911,7 +912,10 @@ class _MessageList extends HookConsumerWidget {
                 final prevMessage = prevEntry?.message;
                 final showDayDivider =
                     prevMessage == null ||
-                    !isSameDay(prevMessage.createdAt, message.createdAt);
+                    !calendarDays.isSameDay(
+                      prevMessage.createdAt,
+                      message.createdAt,
+                    );
 
                 final showAuthor =
                     !message.isSystem &&
@@ -932,7 +936,10 @@ class _MessageList extends HookConsumerWidget {
                     nextMessage == null ||
                     nextMessage.isSystem ||
                     nextMessage.hasAttachments ||
-                    !isSameDay(message.createdAt, nextMessage.createdAt) ||
+                    !calendarDays.isSameDay(
+                      message.createdAt,
+                      nextMessage.createdAt,
+                    ) ||
                     nextMessage.pubkey.toLowerCase() !=
                         message.pubkey.toLowerCase() ||
                     nextMessage.createdAt - message.createdAt > 300;

mobile/lib/features/channels/date_formatters.dart

@@ -79,6 +79,23 @@ bool isSameDay(int a, int b) {
   return dtA.year == dtB.year && dtA.month == dtB.month && dtA.day == dtB.day;
 }
 
+/// Reuses local calendar components while building one timeline.
+///
+/// Adjacent rows compare the same timestamps for dividers, author grouping,
+/// avatars, and sticky headers. Resolve each timestamp only once. Create a new
+/// instance on each build so a timezone change cannot leave stale cached days.
+class TimelineCalendarDays {
+  final _days = <int, (int, int, int)>{};
+
+  (int, int, int) _day(int unixSeconds) => _days.putIfAbsent(unixSeconds, () {
+    final date = DateTime.fromMillisecondsSinceEpoch(unixSeconds * 1000);
+    return (date.year, date.month, date.day);
+  });
+
+  /// Whether two timestamps fall on the same local calendar day.
+  bool isSameDay(int a, int b) => _day(a) == _day(b);
+}
+
 /// Returns a compact relative time string like "just now", "5m ago", "3h ago",
 /// "2d ago", or a short date for older timestamps.
 String relativeTime(int unixSeconds) {

mobile/lib/features/channels/thread_detail_page/message_list.dart

@@ -144,6 +144,7 @@ class _ThreadMessageList extends HookWidget {
 
   @override
   Widget build(BuildContext context) {
+    final calendarDays = TimelineCalendarDays();
     final headLink = useMemoized(LayerLink.new);
     final loadingHeadLink = useMemoized(LayerLink.new);
     final headScrollPosition = useRef<ScrollPosition?>(null);
@@ -237,7 +238,7 @@ class _ThreadMessageList extends HookWidget {
                             ? replies[chronologicalIndex - 1]
                             : null;
                         final previousMessage = previousReply ?? head;
-                        final showDayDivider = !isSameDay(
+                        final showDayDivider = !calendarDays.isSameDay(
                           previousMessage.createdAt,
                           reply.createdAt,
                         );
@@ -258,7 +259,10 @@ class _ThreadMessageList extends HookWidget {
                             reply.hasAttachments ||
                             nextReply?.hasAttachments == true ||
                             nextReply == null ||
-                            !isSameDay(reply.createdAt, nextReply.createdAt) ||
+                            !calendarDays.isSameDay(
+                              reply.createdAt,
+                              nextReply.createdAt,
+                            ) ||
                             nextReply.pubkey.toLowerCase() !=
                                 reply.pubkey.toLowerCase() ||
                             nextReply.createdAt - reply.createdAt > 300;

mobile/test/features/channels/date_formatters_test.dart

@@ -5,6 +5,26 @@ import 'package:buzz/features/channels/date_formatters.dart';
 int _ts(DateTime local) => local.millisecondsSinceEpoch ~/ 1000;
 
 void main() {
+  test('timeline calendar grouping matches uncached local-day comparisons', () {
+    final days = TimelineCalendarDays();
+    final timestamps = [
+      DateTime(2025, 12, 31, 23, 59),
+      DateTime(2026, 1, 1),
+      DateTime(2026, 3, 8, 1, 30),
+      DateTime(2026, 3, 8, 3, 30),
+      DateTime(2026, 11, 1, 1, 30),
+      DateTime(2026, 11, 1, 3, 30),
+      DateTime(2026, 11, 2),
+    ].map(_ts).toList();
+    for (var repeat = 0; repeat < 3; repeat++) {
+      for (final a in timestamps) {
+        for (final b in timestamps) {
+          expect(days.isSameDay(a, b), isSameDay(a, b));
+        }
+      }
+    }
+  });
+
   group('formatDayHeading', () {
     // Fix "now" so tests are deterministic.
     final now = DateTime(2026, 4, 23, 14, 30); // Apr 23 2026, 2:30 PM local