nix : add nix-nodejs facilities to build Web UI (#23846)

nix : add nix-nodejs facilities to build Web UI (#23846)

* nix: add nix-nodejs facilities to build Web UI

Build the Web UI locally using standard Nix systems for building NodeJS
packages.

- Create derivation for the web UI
- npm dependencies are imported via buildNodeModules. Does not require
  setting any shasum.
- Copy build artifacts to the correct folders.
- Prevents having to download from huggingface.co

Fixes #23067

* nix: simplify webui derivation using LLAMA_UI_OUT_DIR

- Move npm build to installPhase with LLAMA_UI_OUT_DIR=$out to write
  output directly to the Nix store
- Copy built assets to tools/ui/dist (source tree) instead of
  build/tools/ui/dist so CMake's copy_src_dist() finds them