feat: scaffolding for Llama Stack UI (#2149)

# What does this PR do?
Introduces scaffolding for Llama Stack's UI. Created with next.js and
https://ui.shadcn.com/.

1. Initialized directory with `npx shadcn@latest init`
2. Added sidebar component `npx shadcn@latest add sidebar` and added
menu items for chat completions and responses.
3. Placeholder pages for each.

## Test Plan
`npm run dev`

<img width="1058" alt="image"
src="https://github.com/user-attachments/assets/5695a53f-e22e-418e-80d1-5bf0ae9b6fe8"
/>

llama_stack/ui/package.json

@@ -0,0 +1,36 @@
+{
+  "name": "ui",
+  "version": "0.1.0",
+  "private": true,
+  "scripts": {
+    "dev": "next dev --turbopack",
+    "build": "next build",
+    "start": "next start",
+    "lint": "next lint"
+  },
+  "dependencies": {
+    "@radix-ui/react-dialog": "^1.1.13",
+    "@radix-ui/react-separator": "^1.1.6",
+    "@radix-ui/react-slot": "^1.2.2",
+    "@radix-ui/react-tooltip": "^1.2.6",
+    "class-variance-authority": "^0.7.1",
+    "clsx": "^2.1.1",
+    "lucide-react": "^0.510.0",
+    "next": "15.3.2",
+    "react": "^19.0.0",
+    "react-dom": "^19.0.0",
+    "tailwind-merge": "^3.3.0"
+  },
+  "devDependencies": {
+    "@eslint/eslintrc": "^3",
+    "@tailwindcss/postcss": "^4",
+    "@types/node": "^20",
+    "@types/react": "^19",
+    "@types/react-dom": "^19",
+    "eslint": "^9",
+    "eslint-config-next": "15.3.2",
+    "tailwindcss": "^4",
+    "tw-animate-css": "^1.2.9",
+    "typescript": "^5"
+  }
+}