notifications-tray-icon: add dedicated module and update flake input

Add notifications-tray-icon.nix module with overlay and home-manager
service config. Update flake input to colonelpanic8 fork and remove
stale HLS input follows. Clean up xmonad.nix by removing the old
overlay reference and commented-out service definition.

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

nixos/configuration.nix

@@ -28,6 +28,7 @@
     ./laptop.nix
     ./nix.nix
     ./nixified.ai.nix
+    ./notifications-tray-icon.nix
     ./nvidia.nix
     ./options.nix
     ./plasma.nix