Add systemd timers to auto-mute/unmute when idle-inhibit is active

Mutes audio at 21:00 and unmutes at 07:00, but only when the
idle-inhibit toggle is active (PID file exists). This is useful
for leaving the computer running overnight for SSH access while
keeping it quiet for sleeping.

systemd/user/auto-mute.timer

@@ -0,0 +1,9 @@
+[Unit]
+Description=Mute audio at 21:00
+
+[Timer]
+OnCalendar=*-*-* 21:00:00
+Persistent=false
+
+[Install]
+WantedBy=timers.target