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.lock

@@ -36,6 +36,26 @@
         "type": "github"
       }
     },
+    "flake-parts": {
+      "inputs": {
+        "nixpkgs-lib": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1775087534,
+        "narHash": "sha256-91qqW8lhL7TLwgQWijoGBbiD4t7/q75KTi8NxjVmSmA=",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "rev": "3107b77cd68437b9a76194f0f7f9c55f2329ca5b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "type": "github"
+      }
+    },
     "flake-utils": {
       "inputs": {
         "systems": "systems"
@@ -281,6 +301,7 @@
     "root": {
       "inputs": {
         "disko": "disko",
+        "flake-parts": "flake-parts",
         "home-manager": "home-manager",
         "nix-darwin": "nix-darwin",
         "nix-openclaw": "nix-openclaw",