fix: use correct icons for mark read/unread action

If any selected room is not marked unread
(anySelectedRoomNotMarkedUnread), then show the 'mark unread' action
icon, else show the 'mark as read' icon.

This makes it consistent with the notification on/off action.

lib/pages/chat_list/chat_list_header.dart

@@ -138,8 +138,8 @@ class ChatListHeader extends StatelessWidget implements PreferredSizeWidget {
                     tooltip: L10n.of(context)!.toggleUnread,
                     icon: Icon(
                       controller.anySelectedRoomNotMarkedUnread
-                          ? Icons.mark_chat_read_outlined
-                          : Icons.mark_chat_unread_outlined,
+                          ? Icons.mark_chat_unread_outlined
+                          : Icons.mark_chat_read_outlined,
                     ),
                     onPressed: controller.toggleUnread,
                   ),