feat(npm): use npmmirror for npm fetches

Set the user-level npm/pnpm registry to npmmirror, and make Nix builds use the same mirror through nixpkgs' npmRegistryOverrides and rewriteURL. The configured Nixpkgs policy is also applied to myNurPkgs and Colmena's nodeNixpkgs so NUR packages such as dsh use the mirror while preserving Colmena parity.

outputs/default.nix

@@ -9,6 +9,22 @@ let
   mylib = import ../lib { inherit lib; };
   myvars = import ../vars { inherit lib; };
 
+  # Keep npm fetches on the domestic mirror, including inside Nix builds.
+  # `rewriteURL` covers plain fetchurl calls; `npmRegistryOverrides` covers
+  # fetchNpmDeps (package-lock tarballs and registry packuments).
+  nixpkgsConfig = {
+    allowUnfree = true;
+    npmRegistryOverrides = {
+      "registry.npmjs.org" = "https://registry.npmmirror.com";
+    };
+    rewriteURL =
+      url:
+      if lib.hasPrefix "https://registry.npmjs.org/" url then
+        "https://registry.npmmirror.com/${lib.removePrefix "https://registry.npmjs.org/" url}"
+      else
+        url;
+  };
+
   # Add my custom lib, vars, nixpkgs instance, and all the inputs to specialArgs,
   # so that I can use them in all my nixos/home-manager/darwin modules.
   genSpecialArgs =
@@ -29,6 +45,7 @@ let
         inputs
         mylib
         myvars
+        nixpkgsConfig
         pkgs-stable
         ;
 
@@ -58,7 +75,14 @@ let
       };
       # NUR packages for this system; modules use `myNurPkgs.dsh` instead of
       # repeating `nur-ryan4yin.packages.${pkgs.stdenv.hostPlatform.system}`.
-      myNurPkgs = inputs.nur-ryan4yin.packages.${system};
+      # Instantiate them with the same Nixpkgs policy as the host so npm
+      # fetchers inside NUR packages also use the mirror.
+      myNurPkgs = import "${inputs.nur-ryan4yin}" {
+        pkgs = import inputs.nixpkgs {
+          inherit system;
+          config = nixpkgsConfig;
+        };
+      };
     };
 
   # This is the args for all the haumea modules in this folder.
@@ -106,7 +130,11 @@ let
     meta = {
       nixpkgs = import nixpkgs { system = "x86_64-linux"; };
       nodeNixpkgs = builtins.mapAttrs (
-        name: _: import nixpkgs { inherit (nixosConfigurations.${name}.pkgs.stdenv.hostPlatform) system; }
+        name: _:
+        import nixpkgs {
+          inherit (nixosConfigurations.${name}.pkgs.stdenv.hostPlatform) system;
+          config = nixpkgsConfig;
+        }
       ) colmenaDeployments;
       nodeSpecialArgs = builtins.mapAttrs (
         name: _: nixosConfigurations.${name}._module.specialArgs