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-volume-default

@@ -0,0 +1,25 @@
+#!/bin/sh
+
+set -eu
+
+
+# Loaded from ~/.config/environment.d/50-defaults.conf or a local override
+VOLUME="${DEFAULT_VOLUME:?unset, check ~/.config/environment.d/}"
+
+
+i=0
+while ! wpctl status >/dev/null 2>&1; do
+    i=$((i + 1))
+    if [ "$i" -ge 20 ]; then
+      echo "no PipeWire sink after 10s" >&2
+      exit 1
+    fi
+    sleep 0.5
+done
+
+
+wpctl set-volume @DEFAULT_AUDIO_SINK@ "$VOLUME%"
+wpctl set-mute @DEFAULT_AUDIO_SINK@ 0
+
+
+echo "Set volume to $VOLUME%"