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/daniel-macbook-air.nix

@@ -0,0 +1,31 @@
+{ inputs, ... }: {
+  flake.darwinConfigurations."Daniel-Macbook-Air" = inputs.nix-darwin.lib.darwinSystem {
+    modules = [
+      # Overlay: make zen-browser available as pkgs.zen-browser
+      { nixpkgs.overlays = [ (final: prev: {
+          zen-browser = inputs.zen-browser.packages.${final.stdenv.hostPlatform.system}.default;
+        }) ];
+      }
+
+      ../hosts/daniel-macbook-air.nix
+      ../fish.nix
+
+      # Home Manager on macOS
+      inputs.home-manager.darwinModules.home-manager
+      ({ lib, ... }: {
+        home-manager.useGlobalPkgs = true;
+        home-manager.useUserPackages = true;
+        # Automatically backup files before home-manager overwrites them
+        home-manager.backupFileExtension = "backup";
+        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
+          ];
+        };
+      })
+    ];
+  };
+}