simplify: dedupe boilerplate and remove dead code across all layers

lib/view/pages/talk/data/chat_message.dart

@@ -16,8 +16,6 @@ class ChatMessage {
   RichObjectString? file;
   String content = '';
 
-  bool get containsFile => file != null;
-
   ChatMessage({required this.originalMessage, this.originalData}) {
     if (originalData?.containsKey('file') ?? false) {
       file = originalData?['file'];