Set default volume and brightness at login

Add three `systemd` user units that reset
volume, screen brightness, and keyboard backlight,
so that every session starts from a known state.

Values come from ~/.config/environment.d/50-defaults.conf,
and can be overridden per machine via an untracked 90-local.conf

.config/systemd/user/set-volume-default.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=Set default output volume
+After=wireplumber.service
+PartOf=graphical-session.target
+Wants=wireplumber.service
+
+[Service]
+Type=oneshot
+ExecStart=%h/.local/bin/set-volume-default
+TimeoutStartSec=30
+
+[Install]
+WantedBy=graphical-session.target