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/tsconfig.json

@@ -0,0 +1,27 @@
+{
+  "compilerOptions": {
+    "target": "ES2017",
+    "lib": ["dom", "dom.iterable", "esnext"],
+    "allowJs": true,
+    "skipLibCheck": true,
+    "strict": true,
+    "noEmit": true,
+    "esModuleInterop": true,
+    "module": "esnext",
+    "moduleResolution": "bundler",
+    "resolveJsonModule": true,
+    "isolatedModules": true,
+    "jsx": "preserve",
+    "incremental": true,
+    "plugins": [
+      {
+        "name": "next"
+      }
+    ],
+    "paths": {
+      "@/*": ["./*"]
+    }
+  },
+  "include": ["next-env.d.ts", "**/*.ts", "**/*.tsx", ".next/types/**/*.ts"],
+  "exclude": ["node_modules"]
+}