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/bin/cli.js

@@ -0,0 +1,34 @@
+#!/usr/bin/env node
+
+const { spawn } = require('child_process');
+const path = require('path');
+
+const port = process.env.LLAMA_STACK_UI_PORT || 8322;
+const uiDir = path.resolve(__dirname, '..');
+const serverPath = path.join(uiDir, '.next', 'standalone', 'ui', 'src', 'llama_stack_ui', 'server.js');
+const serverDir = path.dirname(serverPath);
+
+console.log(`Starting Llama Stack UI on http://localhost:${port}`);
+
+const child = spawn(process.execPath, [serverPath], {
+  cwd: serverDir,
+  stdio: 'inherit',
+  env: {
+    ...process.env,
+    PORT: port,
+  },
+});
+
+process.on('SIGINT', () => {
+  child.kill('SIGINT');
+  process.exit(0);
+});
+
+process.on('SIGTERM', () => {
+  child.kill('SIGTERM');
+  process.exit(0);
+});
+
+child.on('exit', (code) => {
+  process.exit(code);
+});