Offline mode doesn't work when saving objects

When saving objects, Offline mode doesn't work since it still tries to use the `save_file` method in the Offline factory:

```
  File "/home/wk9874/Documents/temp/venv/lib/python3.10/site-packages/simvue/factory/proxy/offline.py", line 138, in save_file
    fh.write(data["pickled"])
TypeError: a bytes-like object is required, not 'str'
```