msglist: Follow user settings for message timestamps

Earlier, timestamps were always formatted as 12-hour (h:mm aa),
ignoring the user's preference `twentyFourHourTime` in settings.

Now, the code retrieves the user's setting and selects the appropriate
format:
• 24-hour format: "HH:mm"
• 12-hour format: "h:mm aa"

Author: Gaurav-Kushwaha-1225

lib/widgets/message_list.dart

@@ -1334,7 +1334,10 @@ class MessageWithPossibleSender extends StatelessWidget {
 
     Widget? senderRow;
     if (item.showSender) {
-      final time = _kMessageTimestampFormat
+      final userSettings = store.userSettings;
+      final is24HourFormat = userSettings?.twentyFourHourTime;
+      final timeFormat = DateFormat(getTimeFormat(is24HourFormat ?? false), ZulipLocalizations.of(context).localeName);
+      final time = timeFormat
         .format(DateTime.fromMillisecondsSinceEpoch(1000 * message.timestamp));
       senderRow = Row(
         mainAxisAlignment: MainAxisAlignment.spaceBetween,
@@ -1438,4 +1441,6 @@ class MessageWithPossibleSender extends StatelessWidget {
 }
 
 // TODO(i18n): web seems to ignore locale in formatting time, but we could do better
-final _kMessageTimestampFormat = DateFormat('h:mm aa', 'en_US');
+String getTimeFormat(bool is24Hour) {
+  return is24Hour ? 'HH:mm' : 'h:mm aa';
+}