Update service.py

Author: programminx-askui

src/askui/chat/api/messages/service.py

@@ -96,7 +96,7 @@ def save(self, thread_id: ThreadId, messages: list[Message]) -> None:
             raise ValueError(error_msg)
         messages = sorted(messages, key=lambda m: m.created_at)
         thread_path = self._get_thread_path(thread_id)
-        with thread_path.open("w") as f:
+        with thread_path.open("w", encoding="utf-8") as f:
             for msg in messages:
                 f.write(msg.model_dump_json())
                 f.write("\n")