muxg: ssh prompts through an askpass popup

open   by a73x

A TUI wall dial's ssh never touches `/dev/tty`: `SSH_ASKPASS_REQUIRE=force` plus `mux askpass` carry a host-key or password prompt to a popup, and a decline SIGTERMs the ssh and ends the tile with `[prompt declined]`. `muxg` has no askpass surface (`grep -ri askpass src/gui/` is empty). An ssh that wants a passphrase or a first-time host-key confirmation has no terminal to ask on and hangs or fails, and the native ssh test (`test/native_ssh.py`) records attempts, not outcomes.

Wanted: the GUI arms `MUX_ASKPASS_SOCK` the way `wall_pump.askOn` does and renders the prompt in a modal; decline ends the pane with the same notice. Reuse `client.askpass`. A gate on retiring the wall.