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
<!-- Describe the tests you ran to verify your changes with result
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-copy-to-clipboard.ts

@@ -0,0 +1,36 @@
+import { useCallback, useRef, useState } from "react"
+import { toast } from "sonner"
+
+type UseCopyToClipboardProps = {
+  text: string
+  copyMessage?: string
+}
+
+export function useCopyToClipboard({
+  text,
+  copyMessage = "Copied to clipboard!",
+}: UseCopyToClipboardProps) {
+  const [isCopied, setIsCopied] = useState(false)
+  const timeoutRef = useRef<NodeJS.Timeout | null>(null)
+
+  const handleCopy = useCallback(() => {
+    navigator.clipboard
+      .writeText(text)
+      .then(() => {
+        toast.success(copyMessage)
+        setIsCopied(true)
+        if (timeoutRef.current) {
+          clearTimeout(timeoutRef.current)
+          timeoutRef.current = null
+        }
+        timeoutRef.current = setTimeout(() => {
+          setIsCopied(false)
+        }, 2000)
+      })
+      .catch(() => {
+        toast.error("Failed to copy to clipboard.")
+      })
+  }, [text, copyMessage])
+
+  return { isCopied, handleCopy }
+}