I think the safest offline storage is to use the browser file system (https://developer.mozilla.org/en-US/docs/Web/API/File_System...) so the data is out of the browser's hands.

It works pretty well with the only downside being the user needs to re-select the file/directory to use each time they use the app and there is no way to force the file picker to start in a specific directory other than the "well known directories".