Solipsis desktop is this site, installed — it ships no separate code. Your browser does it in one click (the install button in Chromium, or File → Add to Dock in Safari): a real dock icon, no browser bar, and notifications when a sealed message lands. Signed Mac and Windows downloads are not published yet.
Moving from Slack? Nothing there waits on a download. The import runs in your browser: channels, messages, threads, timestamps, authors, formatting and emoji come across, and so do canvases and files, as far as Slack will still serve them. Slack Lists are the exception — their contents are not in a Slack export and Slack will not hand them over, so no app can bring them.