flake: update Stylix to master and scope import to desktop hosts

Switch Stylix from release-25.11 to master, which fixes hyprpaper's
wallpaper config generation for 0.8.4 (structured attributes instead
of raw string concatenation).

Move the Stylix module import from common.nix to flake.nix extraModules
so the Mediaserver (on nixpkgs-stable) doesn't pull in options that
only exist in unstable nixpkgs (services.displayManager.generic).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

common.nix

@@ -11,7 +11,6 @@
     # Generic settings #
     ./settings/desktop.nix
     ./settings/hyprland.nix
-    ./settings/stylix.nix
     ./settings/locale.nix
     ./settings/audio.nix
     ./settings/users.nix