{
  description = "Puppy Tracker — offline-first puppy tracking app with server-side sync.";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    let
      mkStatic = pkgs:
        pkgs.stdenvNoCC.mkDerivation {
          pname = "puppy-tracker-static";
          version = "0.2.0";
          src = ./src;
          dontConfigure = true;
          dontBuild = true;
          installPhase = ''
            mkdir -p $out/share/puppy-tracker
            cp -r ./* $out/share/puppy-tracker/
          '';
        };

      mkServer = pkgs:
        pkgs.buildGoModule {
          pname = "puppy-tracker-server";
          version = "0.2.0";
          src = ./server;
          vendorHash = null; # no external dependencies
          # Pure-Go build for a tiny static binary.
          env.CGO_ENABLED = "0";
          ldflags = [ "-s" "-w" ];
          # The binary built from `module puppy-tracker` is `puppy-tracker`,
          # rename so the executable name reflects its role.
          postInstall = ''
            mv $out/bin/puppy-tracker $out/bin/puppy-tracker-server
          '';
          meta.mainProgram = "puppy-tracker-server";
        };
    in
    {
      nixosModules.default = import ./module.nix self;
      nixosModules.puppy-tracker = self.nixosModules.default;
    }
    //
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = import nixpkgs { inherit system; };
        static = mkStatic pkgs;
        server = mkServer pkgs;
      in
      {
        packages.static = static;
        packages.server = server;
        # `default` bundles both so `nix build` produces a runnable directory.
        packages.default = pkgs.symlinkJoin {
          name = "puppy-tracker";
          paths = [ server static ];
        };

        devShells.default = pkgs.mkShell {
          packages = [ pkgs.go pkgs.python3 ];
          shellHook = ''
            echo "puppy-tracker dev shell"
            echo "  cd server && go run . -static ../src -data /tmp/puppy-events.json"
          '';
        };

        # `nix run` → start the server, serving the bundled static files,
        # storing data in $XDG_DATA_HOME/puppy-tracker/events.json.
        apps.default = {
          type = "app";
          program = toString (pkgs.writeShellScript "puppy-tracker-run" ''
            data_dir="''${XDG_DATA_HOME:-$HOME/.local/share}/puppy-tracker"
            mkdir -p "$data_dir"
            exec ${server}/bin/puppy-tracker-server \
              -addr "''${PUPPY_ADDR:-:8080}" \
              -static ${static}/share/puppy-tracker \
              -data "$data_dir/events.json"
          '');
          meta.description = "Run puppy-tracker locally (data in $XDG_DATA_HOME/puppy-tracker)";
        };
      }
    );
}