feat: 新增 laptop 主机配置（Ultra 9 285H / Arc）

- hosts/laptop/hardware.nix: 本机真实硬件配置（btrfs 子卷 / NPU / 微码）
- hosts/laptop/default.nix: 复用 common/desktop-plasma/chinese/gaming/printing/apps，跳过 nvidia 与 fpga；加 Intel Arc VA-API
- flake.nix: 抽出 mkSystem，新增 nixosConfigurations.laptop / nixos

flake.nix

@@ -1,5 +1,5 @@
 {
-  description = "whc NixOS — desktop (i7-12700F + RTX 4060)";
+  description = "whc NixOS — desktop (i7-12700F + RTX 4060) + laptop (Ultra 9 285H / Arc)";
 
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
@@ -9,11 +9,17 @@
     { self, nixpkgs, ... }:
     let
       system = "x86_64-linux";
+      mkSystem = host: nixpkgs.lib.nixosSystem {
+        inherit system;
+        modules = [ (./hosts + "/${host}") ];
+      };
     in
     {
-      nixosConfigurations.desktop = nixpkgs.lib.nixosSystem {
-        inherit system;
-        modules = [ ./hosts/desktop ];
+      nixosConfigurations = {
+        desktop = mkSystem "desktop";
+        laptop = mkSystem "laptop";
+        # hostname 仍是 nixos：让不带 attr 的 `nixos-rebuild --flake .#` 也能命中
+        nixos = mkSystem "laptop";
       };
 
       # 无 flake 时也可：把本仓库拷到 /etc/nixos 后 nixos-rebuild switch