chore(ui): add npm package and dockerfile (#4100)

# What does this PR do?
- sets up package.json for npm `llama-stack-ui` package (will update
llama-stack-ops)
- adds dockerfile for UI docker image

## Test Plan
npx:
npm build && npm pack
LLAMA_STACK_UI_PORT=8322 npx
/Users/erichuang/projects/ui/src/llama_stack_ui/llama-stack-ui-0.4.0-alpha.2.tgz

docker:
cd src/llama_stack_ui
docker build . -f Dockerfile  --tag test_ui --no-cache

❯ docker run -p 8322:8322 \
      -e LLAMA_STACK_UI_PORT=8322 \
      test_ui:latest

src/llama_stack_ui/scripts/postbuild.js

@@ -0,0 +1,40 @@
+const fs = require('fs');
+const path = require('path');
+
+// Copy public directory to standalone
+const publicSrc = path.join(__dirname, '..', 'public');
+const publicDest = path.join(__dirname, '..', '.next', 'standalone', 'ui', 'src', 'llama_stack_ui', 'public');
+
+if (fs.existsSync(publicSrc) && !fs.existsSync(publicDest)) {
+  console.log('Copying public directory to standalone...');
+  copyDir(publicSrc, publicDest);
+}
+
+// Copy .next/static to standalone
+const staticSrc = path.join(__dirname, '..', '.next', 'static');
+const staticDest = path.join(__dirname, '..', '.next', 'standalone', 'ui', 'src', 'llama_stack_ui', '.next', 'static');
+
+if (fs.existsSync(staticSrc) && !fs.existsSync(staticDest)) {
+  console.log('Copying .next/static to standalone...');
+  copyDir(staticSrc, staticDest);
+}
+
+function copyDir(src, dest) {
+  if (!fs.existsSync(dest)) {
+    fs.mkdirSync(dest, { recursive: true });
+  }
+
+  const files = fs.readdirSync(src);
+  files.forEach((file) => {
+    const srcFile = path.join(src, file);
+    const destFile = path.join(dest, file);
+
+    if (fs.statSync(srcFile).isDirectory()) {
+      copyDir(srcFile, destFile);
+    } else {
+      fs.copyFileSync(srcFile, destFile);
+    }
+  });
+}
+
+console.log('Postbuild complete!');