Use deeplinks for desktop login instead of localhost server

[Brendonovich]

Desktop login is current done using tauri-plugin-oauth, which spawns a localhost server that the web login process redirects to. This isn't 100% reliable, a number of users have had the localhost server just not work.

Redirect to a cap-desktop:// deeplink should be more reliable, this will require work in both desktop and web to perform and handle the new redirect.

Deeplinks don't work with tauri dev on macos so we'll probably want to support the old flow in web in perpetuity and just during dev in desktop.