Route the volume and brightness keys through swayosd-client, which raises
a transient on-screen popup that auto-hides a moment after the last press,
so the level is visible while adjusting without a permanent widget. A new
osd module runs the server for the graphical session and makes the
backlight node group-writable so it dims without root.
Supersedes the plain brightnessctl binds, whose control swayosd bundles.
