chore: clean up and optimize nixos configuration

- fix: nvidia.nix missing semicolon (build error)
- remove: ranger, vim from core/pkg.nix (yazi + neovim covers)
- remove: htop duplicate in home/pkg.nix (keep in system)
- replace: neofetch with fastfetch in home/pkg.nix
- remove: nixd LSP, keep nil only in nvchad.nix
- remove: empty server/pkg.nix and no-op server/firewall.nix
- remove: redundant services.pulseaudio.enable = false default
- add: comprehensive README.md documenting architecture

modules/server/default.nix

@@ -1,8 +1,6 @@
 { ... }:
 {
   imports = [
-    ./pkg.nix
-    ./firewall.nix
     ./docker.nix
     ./ssh.nix
   ];