refactor(nix): migrate to flake-parts, drop specialArgs ♻️

- Convert flake.nix to flake-parts.lib.mkFlake; split each host into
  its own module under nixos/flake-modules/.
- Replace zen-browser specialArgs plumbing with a nixpkgs overlay so
  home.nix can just reference pkgs.zen-browser.

nixos/flake-modules/wsl.nix

@@ -0,0 +1,24 @@
+{ inputs, ... }: {
+  flake.nixosConfigurations.wsl = inputs.nixpkgs.lib.nixosSystem {
+    system = "x86_64-linux";
+    modules = [
+      inputs.nixos-wsl.nixosModules.default
+      inputs.vscode-server.nixosModules.default
+      ../hosts/wsl.nix
+      ../fish.nix
+
+      # Home Manager on WSL
+      inputs.home-manager.nixosModules.home-manager
+      ({ lib, ... }: {
+        home-manager.useGlobalPkgs = true;
+        home-manager.useUserPackages = true;
+        home-manager.backupFileExtension = "backup";
+        home-manager.users.dth = { ... }: {
+          home.username = "dth";
+          home.homeDirectory = lib.mkForce "/home/dth";
+          imports = [ ../home/danny/home.nix ];
+        };
+      })
+    ];
+  };
+}