Extract the per-host home-manager block (useGlobalPkgs, useUserPackages,
backupFileExtension, users.<name> with username/homeDirectory/optional
stateVersion/optional imports) into nixos/lib/home-manager-user.nix.
Each flake-module now imports it with its per-host parameters, removing
~40 lines of boilerplate across the four hosts.
- 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.