Login Flow v2 cannot complete without a browser, and the launcher had a branch for xdg-open, one for Android's Intent, and an honest Unsupported error for everything else — which on Windows stranded the flow on "approve the sign-in in your browser". rundll32 url.dll,FileProtocolHandler is ShellExecute on the URL and needs no crate; chosen over cmd /C start, whose quoting of & in a query string is a known trap. Not verified: Wine has no browser to open.