feat: swayosd on-screen display for volume/brightness/mute

The media keys called wpctl/brightnessctl directly with no visual
feedback — changing volume showed nothing on screen. Now they drive
swayosd-client, which performs the action AND shows a themed OSD.

- modules/home/osd.nix: services.swayosd, style.css generated from the
  palette (base bg, accent progress/border); nomarchy.osd.enable.
- hyprland media binds → swayosd-client (--output-volume raise/lower/
  mute-toggle, --input-volume mute-toggle, --brightness raise/lower).
- modules/nixos: services.udev.packages = [ swayosd ] installs the
  backlight rule (chgrp video + g+w) so brightness works for video-group
  users without root (the installer/live user is in video).

swaync is unaffected (it was already running — this was purely the
missing OSD). Verified: flake check green, swayosd.service bound to
graphical-session.target, hyprland config verifies, swayosd in the live
ISO closure. On-screen rendering needs a real session to confirm.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

modules/home/default.nix

@@ -16,6 +16,7 @@
     ./swaync.nix    # notification daemon, themed from the same JSON
     ./idle.nix      # hyprlock + hypridle, themed from the same JSON
     ./yazi.nix      # flagship TUI file manager, themed + plugins
+    ./osd.nix       # swayosd volume/brightness OSD, themed
   ];
 
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