fix: remove npm install for demo assets (no package.json needed)

The demo application doesn't have npm dependencies - it uses esbuild
and tailwind which are managed by Mix. The npm install step was
causing the build to fail since demo/assets has no package.json.

Author: ukutaht

demo/Dockerfile

@@ -69,9 +69,6 @@ COPY demo/lib lib
 
 COPY demo/assets assets
 
-# install npm dependencies
-RUN npm install --prefix ./assets
-
 # compile assets
 RUN mix assets.deploy