Switch musicdolphin's screen from the IR remote's power key

pi_kiosk installs kiosk-screen on|off|toggle|status, which switches the monitor via
DPMS on the kiosk's X display (the Dell P2419H has no HDMI-CEC). Turning it back on
also turns DPMS off again, as .xinitrc does, so the screen never blanks by itself.

musicdolphin's backend config runs `kiosk-screen toggle` for the power key (the new
screen: section, MusicMouse 2d0caeb). config.yml is install-once, so the running host
is changed by hand the same way.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

roles/pi_kiosk/templates/kiosk-screen.j2

@@ -0,0 +1,44 @@
+#!/bin/sh
+# Managed by ansible (pi_kiosk role).
+#
+# kiosk-screen on|off|toggle|status - switch the kiosk's monitor on or off.
+#
+# "Off" is DPMS: X stops the HDMI signal and the monitor drops into standby. The
+# session, the browser and everything playing carry on. The MusicMouse backend runs
+# `kiosk-screen toggle` for the IR remote's power key (its config's screen: section);
+# it works over ssh as root too.
+#
+# .xinitrc turns DPMS off so the screen never blanks by itself. Forcing it off has to
+# turn DPMS on, so `on` turns it back off again - otherwise the screen would blank on
+# its own ten minutes after every switch-on.
+
+set -e
+export DISPLAY=:0
+export XAUTHORITY=/home/{{ pi_kiosk_user }}/.Xauthority
+
+# With DPMS disabled `xset q` prints no monitor line at all, and the monitor is on.
+status() {
+    if xset q | grep -q "Monitor is Off"; then echo off; else echo on; fi
+}
+
+screen_on() {
+    xset dpms force on
+    xset -dpms
+}
+
+screen_off() {
+    xset +dpms
+    xset dpms force off
+}
+
+case "$1" in
+    on) screen_on ;;
+    off) screen_off ;;
+    toggle) if [ "$(status)" = off ]; then screen_on; else screen_off; fi ;;
+    status) ;;
+    *)
+        echo "usage: $0 on|off|toggle|status" >&2
+        exit 2
+        ;;
+esac
+status