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

.local/bin/dotfiles-install

@@ -176,6 +176,9 @@ fi
 if systemctl --user daemon-reload 2>/dev/null; then
     systemctl --user enable gnome-restore.service
     systemctl --user enable night-light-reset.timer
+    systemctl --user enable set-brightness-default.service
+    systemctl --user enable set-keyboard-brightness-default.service
+    systemctl --user enable set-volume-default.service
     systemctl --user enable xcape.service
 else
     echo "WARN: no systemd user session, units not enabled" >&2