Doesn't these options already exist as the import and export features in the page https://www.sweethome3d.com/fr/SweetHom ... anager.jsp for identified users?

If you want to remove the use of a server, reading a file from the local disk can be done with HomeRecorder.readHome method with a BlobURLContent built from a file chosen by the user with an [font=courier new]<input type="file">[/font] HTML element (you'll find examples in JSViewFactory.js).
But saving is another story! Even if I recently added the DirectHomeRecorder class to generate a [font=courier new]Home.xml[/font] entry in a ZIP file, there's still no method to generate a SH3X file containing all the resources it needs (images and 3D models), like DefaultHomeOutputStream Java class (a SH3X file is like a SH3D file bu without the old [font=courier new]Home[/font] serialized entry).
By the way, as far as I know, there's no function in JavaScript to write in a local file from a browser. You can write only in local storage or indexedDB.