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

home/nvchad.nix

@@ -8,11 +8,10 @@
   programs.nvchad = {
     enable = true;
 
-    # 可选：添加你需要的语言服务器和工具
+    # NvChad 需要的额外包
     extraPackages = with pkgs; [
       # LSP 服务器
       nil                # Nix LSP
-      nixd               # Nix 语言服务器（另一种选择）
       marksman           # Markdown LSP
 
       # 格式化工具
@@ -26,4 +25,4 @@
     EDITOR = "nvim";
     VISUAL = "nvim";
   };
-}
+}

home/pkg.nix

@@ -4,7 +4,6 @@
     duf
     ncdu
     yazi
-    htop
-    neofetch
+    fastfetch
   ];
-}
+}