To answer your question, standalone desktop that can spit out a Library-of-congress-approved archive format like xml, sqlite, rtf etc.

You could also take the NoteWiki idea which is a simple text control which auto-creates hyperlinks on camelCase words and saves each page as a separate file. Then you could have whatever existing sync solution the user uses (or syncthing) handle backups etc. Having a rich text editor where one can paste images and is saved as a per file format would be ideal for me personally but truthfully I'd like to hear more about what your desired use case is.