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/set-brightness-default

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+set -eu
+
+
+# Loaded from ~/.config/environment.d/50-defaults.conf or a local override
+BRIGHTNESS="${DEFAULT_BRIGHTNESS:?unset, check ~/.config/environment.d/}"
+
+
+gdbus wait --session --timeout 10 org.gnome.SettingsDaemon.Power
+
+gdbus call --session \
+    --dest org.gnome.SettingsDaemon.Power \
+    --object-path /org/gnome/SettingsDaemon/Power \
+    --method org.freedesktop.DBus.Properties.Set \
+    org.gnome.SettingsDaemon.Power.Screen Brightness "<int32 $BRIGHTNESS>" \
+    >/dev/null
+
+
+echo "Set screen brightness to $BRIGHTNESS"