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/options.nix

@@ -67,6 +67,7 @@
     swaync.enable = lib.mkEnableOption "swaync notifications, themed from the state file" // { default = true; };
     idle.enable = lib.mkEnableOption "hyprlock + hypridle (idle lock, display off, suspend)" // { default = true; };
     yazi.enable = lib.mkEnableOption "the yazi TUI file manager, themed with a curated plugin set" // { default = true; };
+    osd.enable = lib.mkEnableOption "swayosd on-screen display for volume/brightness/mute" // { default = true; };
     ghostty.enable = lib.mkEnableOption "Nomarchy's Ghostty configuration" // { default = true; };
     btop.enable = lib.mkEnableOption "btop with the per-theme nomarchy theme" // { default = true; };
     stylix.enable = lib.mkEnableOption "Stylix theming for the long tail of apps (GTK, Qt, cursors)" // { default = true; };