refactored room and file sorting to use direct comparators instead of temporary sort strings, removed obsolete 'sort' properties from API models, and improved file list sorting with case-insensitive name comparisons and null-safe date handling

lib/api/marianumcloud/talk/room/get_room_response.dart

@@ -16,26 +16,20 @@ class GetRoomResponse extends ApiResponse {
   Map<String, dynamic> toJson() => _$GetRoomResponseToJson(this);
 
   List<GetRoomResponseObject> sortBy({
-    bool lastActivity = true,
     required bool favoritesToTop,
     required bool unreadToTop,
   }) {
-    for (var chat in data) {
-      final buffer = StringBuffer();
-
-      if (favoritesToTop) {
-        buffer.write(chat.isFavorite ? 'b' : 'a');
+    return data.toList()..sort((a, b) {
+      if (favoritesToTop && a.isFavorite != b.isFavorite) {
+        return a.isFavorite ? -1 : 1;
       }
       if (unreadToTop) {
-        buffer.write(chat.unreadMessages > 0 ? 'b' : 'a');
+        final aUnread = a.unreadMessages > 0;
+        final bUnread = b.unreadMessages > 0;
+        if (aUnread != bUnread) return aUnread ? -1 : 1;
       }
-
-      buffer.write(chat.lastActivity);
-
-      chat.sort = buffer.toString();
-    }
-
-    return data.toList()..sort((a, b) => b.sort!.compareTo(a.sort!));
+      return b.lastActivity.compareTo(a.lastActivity);
+    });
   }
 }
 
@@ -71,7 +65,6 @@ class GetRoomResponseObject {
   String? status;
   String? statusIcon;
   String? statusMessage;
-  String? sort;
 
   GetRoomResponseObject(
     this.id,