* mux: add herdr backend
* mux: remove herdr service tests
* mux: include herdr in settings subtitle
* mux: add translator context to the multiplexers subtitle
* mux: drive the service from a backend table
Each backend (tmux, zellij, herdr) is one entry in MuxBackends.js with
its commands and parser, so the service no longer branches on muxType
in eight places and the settings dropdown lists the table keys. One
availability check runs for the selected backend and re-runs when the
backend changes. The herdr parser reads the real {sessions:[...]}
shape only.
