feat(UI): adding MVP playground UI (#2828)

# What does this PR do?
I've been tinkering a little with a simple chat playground in the UI, so
I'm opening the PR with what's kind of a WIP.

If you look at the first commit, that includes the big part of the
changes. The rest of the files changed come from adding installing the
`shadcn` components.

Note this is missing a lot; e.g.,
- sessions
- document upload
- audio (the shadcn components install these by default from
https://shadcn-chatbot-kit.vercel.app/docs/components/chat)

I still need to wire up a lot more to make it actually fully functional
but it does basic chat using the LS Typescript Client.

Basic demo: 

<img width="1329" height="1430" alt="Image"
src="https://github.com/user-attachments/assets/917a2096-36d4-4925-b83b-f1f2cda98698"
/>

<img width="1319" height="1424" alt="Image"
src="https://github.com/user-attachments/assets/fab1583b-1c72-4bf3-baf2-405aee13c6bb"
/>


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## Test Plan
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summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->

---------

Signed-off-by: Francisco Javier Arceo <farceo@redhat.com>

llama_stack/ui/components/chat-playground/prompt-suggestions.tsx

@@ -0,0 +1,28 @@
+interface PromptSuggestionsProps {
+  label: string
+  append: (message: { role: "user"; content: string }) => void
+  suggestions: string[]
+}
+
+export function PromptSuggestions({
+  label,
+  append,
+  suggestions,
+}: PromptSuggestionsProps) {
+  return (
+    <div className="space-y-6">
+      <h2 className="text-center text-2xl font-bold">{label}</h2>
+      <div className="flex gap-6 text-sm">
+        {suggestions.map((suggestion) => (
+          <button
+            key={suggestion}
+            onClick={() => append({ role: "user", content: suggestion })}
+            className="h-max flex-1 rounded-xl border bg-background p-4 hover:bg-muted"
+          >
+            <p>{suggestion}</p>
+          </button>
+        ))}
+      </div>
+    </div>
+  )
+}