The Elecraft handling was buried in the Kenwood "data mode" option, so users
couldn't find it. Add "Elecraft K3/K4" as its own entry in the CAT backend
selector. It reuses the Kenwood-dialect transport and the Kenwood USB/network
settings (the K3 emulates the Kenwood command set — a separate transport would be
a near-total duplicate), with an `elecraft` flag on the client that forces
digital modes to DATA A (MD6+DT0) — no data-mode dropdown needed. RigState still
reports Backend "kenwood" so the CW-over-CAT keyer capability keeps working.