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/server-install.nix

@@ -0,0 +1,11 @@
+{ inputs, ... }: {
+  # For disko-install: LUKS + WiFi; hostname/WiFi via --system-config.
+  flake.nixosConfigurations.server-install = inputs.nixpkgs.lib.nixosSystem {
+    system = "x86_64-linux";
+    modules = [
+      inputs.disko.nixosModules.disko
+      ../disko-server.nix
+      ../hosts/server-install.nix
+    ];
+  };
+}