Improve notifications list UX with auto-refresh and friendly timestamps

- Show "Just now" for notifications 2 minutes old or less
- Auto-refresh the list every minute, preserving the active filter

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/lib/utils/friendly_date_formatter.dart

@@ -14,6 +14,7 @@ class FriendlyDateFormatter {
       dateTime.day,
     );
 
+    if (timeSince.inMinutes <= 2) return 'Just now';
     if (timeSince.inMinutes < 60) return '${timeSince.inMinutes} minutes ago';
     if (dateTimeWithoutTime == today) {
       return 'Today at ${DateFormat.Hm().format(dateTime)}';