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/sunken-ship.nix

@@ -0,0 +1,21 @@
+{ inputs, ... }: {
+  flake.nixosConfigurations.sunken-ship = inputs.nixpkgs.lib.nixosSystem {
+    system = "x86_64-linux";
+    modules = [
+      ../hosts/sunken-ship.nix
+
+      # Home Manager on NixOS
+      inputs.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";
+        };
+      })
+    ];
+  };
+}