The native Electron desktop app shipped (PR #20059 and follow-ups) but the
docs only told people how to download it, not what it is or how to use it.
Adds website/docs/user-guide/desktop.md covering install (installer +
prebuilt + Windows GUI), the chat-first UI and management panes, the
hermes desktop CLI flag reference, self-update, how-it-works, and
troubleshooting. Sourced from apps/desktop/README.md, routes.ts, and the
real argparse. Wired into sidebars.ts under Interfaces after the TUI.
