Expand user-guide/multi-connection-desktop.md into a complete setup
walkthrough: where to find the pane (settings nav, profile-rail plug,
command palette), the exact add-connection editor fields (Name,
Gateway URL, Authentication: Session token/OAuth, SSH host), Primary /
This device pills, Test semantics, agent roster + profile-rail
switching and per-profile session/cron/messaging scoping, token
storage via Electron safeStorage with the keyring-less Linux plain-
text opt-in, and troubleshooting. All quoted labels match the desktop
i18n strings. Cross-link the rail entry point from desktop.md.
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