Adds a Nix flake exposing the pad binary as packages.default (buildGoModule
+ importNpmLock for the embedded SvelteKit UI), a devShell, and flake
checks (package build with `go test ./...`, plus a `pad --version` smoke
test). nix/package.nix is written nixpkgs-submission-ready (no
flake-specific inputs) so it can later be adapted for pkgs/by-name.
Also adds a GitHub Actions workflow that runs `nix flake check` and
`nix build` on push/PR, and documents `nix run` / `nix profile install`
/ `nix develop` in the README.
Claude