frontend: this branch bundles only the app. The node carried two frontends and picked one per request: the web one for any browser with an sp_web_beta=1 cookie, which a toggle in the app's advanced settings set for a year. Anyone who had flipped it once kept getting the other interface. The web frontend is no longer compiled into the binary (nothing imports its embed package), the dispatcher serves the app whatever a request carries and expires the old cookie when it sees one, the settings toggle is gone, and --frontend=web is accepted and ignored with a warning rather than failing a node whose config still names it