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/hooks/use-autosize-textarea.ts

@@ -0,0 +1,39 @@
+import { useLayoutEffect, useRef } from "react"
+
+interface UseAutosizeTextAreaProps {
+  ref: React.RefObject<HTMLTextAreaElement | null>
+  maxHeight?: number
+  borderWidth?: number
+  dependencies: React.DependencyList
+}
+
+export function useAutosizeTextArea({
+  ref,
+  maxHeight = Number.MAX_SAFE_INTEGER,
+  borderWidth = 0,
+  dependencies,
+}: UseAutosizeTextAreaProps) {
+  const originalHeight = useRef<number | null>(null)
+
+  useLayoutEffect(() => {
+    if (!ref.current) return
+
+    const currentRef = ref.current
+    const borderAdjustment = borderWidth * 2
+
+    if (originalHeight.current === null) {
+      originalHeight.current = currentRef.scrollHeight - borderAdjustment
+    }
+
+    currentRef.style.removeProperty("height")
+    const scrollHeight = currentRef.scrollHeight
+
+    // Make sure we don't go over maxHeight
+    const clampedToMax = Math.min(scrollHeight, maxHeight)
+    // Make sure we don't go less than the original height
+    const clampedToMin = Math.max(clampedToMax, originalHeight.current)
+
+    currentRef.style.height = `${clampedToMin + borderAdjustment}px`
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [maxHeight, ref, ...dependencies])
+}