muxg: end a session (end_req two-step) from pane and picker

open   by a73x

The native tiling design (2026-09-05) makes detach and end distinct, and `interaction.zig` has detach. Nothing in `src/gui/` sends `end_req` or calls `client.endSession`: the GUI cannot end a session, only leave it. The TUI has two doors (`Ctrl-\ X` through the pane's own connection, picker `x` over a side connection) with the daemon's own others-attached two-step and a 3 s arm.

Wanted: end-from-pane and end-from-picker in the GUI over the same `end_req`/`end_reply` verbs, the refusal shown and a second press within the arm confirming, the pane leaving when the shell does. A gate on retiring the wall.