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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<!-- Closes #[issue-number] -->

## 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/components/chat-playground/message-list.tsx

@@ -0,0 +1,45 @@
+import {
+  ChatMessage,
+  type ChatMessageProps,
+  type Message,
+} from "@/components/chat-playground/chat-message"
+import { TypingIndicator } from "@/components/chat-playground/typing-indicator"
+
+type AdditionalMessageOptions = Omit<ChatMessageProps, keyof Message>
+
+interface MessageListProps {
+  messages: Message[]
+  showTimeStamps?: boolean
+  isTyping?: boolean
+  messageOptions?:
+    | AdditionalMessageOptions
+    | ((message: Message) => AdditionalMessageOptions)
+}
+
+export function MessageList({
+  messages,
+  showTimeStamps = true,
+  isTyping = false,
+  messageOptions,
+}: MessageListProps) {
+  return (
+    <div className="space-y-4 overflow-visible">
+      {messages.map((message, index) => {
+        const additionalOptions =
+          typeof messageOptions === "function"
+            ? messageOptions(message)
+            : messageOptions
+
+        return (
+          <ChatMessage
+            key={index}
+            showTimeStamp={showTimeStamps}
+            {...message}
+            {...additionalOptions}
+          />
+        )
+      })}
+      {isTyping && <TypingIndicator />}
+    </div>
+  )
+}