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.nix

@@ -4,6 +4,9 @@
     nixos-wsl.url = "github:nix-community/NixOS-WSL/main";
     vscode-server.url = "github:nix-community/nixos-vscode-server";
 
+    flake-parts.url = "github:hercules-ci/flake-parts";
+    flake-parts.inputs.nixpkgs-lib.follows = "nixpkgs";
+
     nix-darwin.url = "github:nix-darwin/nix-darwin/master";
     nix-darwin.inputs.nixpkgs.follows = "nixpkgs";
 
@@ -20,131 +23,17 @@
     nix-openclaw.inputs.nixpkgs.follows = "nixpkgs";
   };
 
-  outputs = {
-    nixpkgs,
-    nixos-wsl,
-    vscode-server,
-    nix-darwin,
-    self,
-    home-manager,
-    zen-browser,
-    disko,
-    nix-openclaw,
-    ...
-  }: {
-    nixosConfigurations = {
-      wsl = nixpkgs.lib.nixosSystem {
-        system = "x86_64-linux";
-        modules = [
-          nixos-wsl.nixosModules.default
-          vscode-server.nixosModules.default
-          ./hosts/wsl.nix
-          ./fish.nix
+  outputs = inputs @ { flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
+      systems = [ "x86_64-linux" "aarch64-darwin" ];
 
-          # Home Manager on WSL
-          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 ];
-            };
-          })
-        ];
-      };
-
-      sunken-ship = nixpkgs.lib.nixosSystem {
-        system = "x86_64-linux";
-        modules = [
-          ./hosts/sunken-ship.nix
-
-          # Home Manager on NixOS
-          home-manager.nixosModules.home-manager
-          ({ lib, ... }: {
-            home-manager.useGlobalPkgs = true;
-            home-manager.useUserPackages = true;
-            home-manager.backupFileExtension = "backup";
-            home-manager.users.danny = { ... }: {
-              home.username = "danny";
-              home.homeDirectory = lib.mkForce "/home/danny";
-              home.stateVersion = "25.11";
-            };
-          })
-        ];
-      };
-
-      phantom-ship = nixpkgs.lib.nixosSystem {
-        system = "x86_64-linux";
-        modules = [
-          nix-openclaw.nixosModules.openclaw-gateway
-          ./hosts/phantom-ship.nix
-
-          # Home Manager on NixOS
-          home-manager.nixosModules.home-manager
-          ({ lib, ... }: {
-            home-manager.useGlobalPkgs = true;
-            home-manager.useUserPackages = true;
-            home-manager.backupFileExtension = "backup";
-            home-manager.users.danny = { ... }: {
-              home.username = "danny";
-              home.homeDirectory = lib.mkForce "/home/danny";
-              home.stateVersion = "25.11";
-            };
-          })
-        ];
-      };
-
-      # For disko-install: LUKS + WiFi; hostname/WiFi via --system-config.
-      server-install = nixpkgs.lib.nixosSystem {
-        system = "x86_64-linux";
-        modules = [
-          disko.nixosModules.disko
-          ./disko-server.nix
-          ./hosts/server-install.nix
-        ];
-      };
-
-      # Custom minimal installer ISO (build with: nix build .#installer-iso).
-      # Optional: add ./installer-wifi.nix (gitignored) to modules for live WiFi.
-      installer-iso = nixpkgs.lib.nixosSystem {
-        system = "x86_64-linux";
-        modules = [ ./installer-iso.nix ];
-      };
-    };
-
-    packages.x86_64-linux.installer-iso =
-      self.nixosConfigurations.installer-iso.config.system.build.isoImage;
-
-    # macOS (nix-darwin) configuration
-    darwinConfigurations."Daniel-Macbook-Air" = nix-darwin.lib.darwinSystem {
-      specialArgs = { inherit zen-browser; };
-      modules = [
-        ./hosts/daniel-macbook-air.nix
-        ./fish.nix
-
-        # Home Manager on macOS
-        home-manager.darwinModules.home-manager
-        ({ lib, zen-browser, ... }: {
-          home-manager.useGlobalPkgs = true;
-          home-manager.useUserPackages = true;
-          # Automatically backup files before home-manager overwrites them
-          home-manager.backupFileExtension = "backup";
-          # Pass flake inputs to home-manager modules (e.g. home.nix)
-          home-manager.extraSpecialArgs = { inherit zen-browser; };
-          home-manager.users.danny = { ... }: {
-
-            # Force an absolute path even if another module sets a bad value.
-            home.username = "danny";
-            home.homeDirectory = lib.mkForce "/Users/danny";
-            imports = [
-              ./home/danny/home.nix
-            ];
-          };
-        })
+      imports = [
+        ./flake-modules/wsl.nix
+        ./flake-modules/sunken-ship.nix
+        ./flake-modules/phantom-ship.nix
+        ./flake-modules/daniel-macbook-air.nix
+        ./flake-modules/server-install.nix
+        ./flake-modules/installer-iso.nix
       ];
     };
-  };
 }