chore(repo): add Nix flake for reproducible webui + cli builds

[TheCodedKid]

Summary

Adds a Nix flake so the webui server and cli can be built and run reproducibly with a single command, no manual node/pnpm setup.

• flake.nix — pins nixpkgs, selects node 24 + pnpm 11, wires callPackage.
• pkgs/deemix.nix — the derivation: offline pnpm deps (fetchPnpmDeps), turbo build, installs both runnables.

One shared pnpm-workspace build backs both outputs (they share the workspace deemix + deezer-sdk deps).

Usage

nix build .#webui          # or .#cli, .#default
nix run   .#webui          # server on 0.0.0.0:6595
nix run   .#cli -- <url>
nix develop                # dev shell: node 24 + pnpm 11

Checklist

• I have tested the changes locally and they work as expected
• This PR contains a changeset (pnpm changeset)

[changeset-bot]

🦋 Changeset detected

Latest commit: e0ecc77

The changes in this PR will be included in the next version bump.

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[TheCodedKid]

oops, forgot to include that