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"
/>


<!-- If resolving an issue, uncomment and update the line below -->
<!-- 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/layout/app-sidebar.tsx

@@ -5,6 +5,7 @@ import {
   MessagesSquare,
   MoveUpRight,
   Database,
+  MessageCircle,
 } from "lucide-react";
 import Link from "next/link";
 import { usePathname } from "next/navigation";
@@ -21,7 +22,14 @@ import {
   SidebarMenuItem,
   SidebarHeader,
 } from "@/components/ui/sidebar";
+// Extracted Chat Playground item
+const chatPlaygroundItem = {
+  title: "Chat Playground",
+  url: "/chat-playground",
+  icon: MessageCircle,
+};
 
+// Removed Chat Playground from log items
 const logItems = [
   {
     title: "Chat Completions",
@@ -54,6 +62,35 @@ export function AppSidebar() {
         <Link href="/">Llama Stack</Link>
       </SidebarHeader>
       <SidebarContent>
+        {/* Chat Playground as its own section */}
+        <SidebarGroup>
+          <SidebarGroupContent>
+            <SidebarMenu>
+              <SidebarMenuItem>
+                <SidebarMenuButton
+                  asChild
+                  className={cn(
+                    "justify-start",
+                    pathname.startsWith(chatPlaygroundItem.url) &&
+                      "bg-gray-200 dark:bg-gray-700 hover:bg-gray-200 dark:hover:bg-gray-700 text-gray-900 dark:text-gray-100",
+                  )}
+                >
+                  <Link href={chatPlaygroundItem.url}>
+                    <chatPlaygroundItem.icon
+                      className={cn(
+                        pathname.startsWith(chatPlaygroundItem.url) && "text-gray-900 dark:text-gray-100",
+                        "mr-2 h-4 w-4",
+                      )}
+                    />
+                    <span>{chatPlaygroundItem.title}</span>
+                  </Link>
+                </SidebarMenuButton>
+              </SidebarMenuItem>
+            </SidebarMenu>
+          </SidebarGroupContent>
+        </SidebarGroup>
+
+        {/* Logs section */}
         <SidebarGroup>
           <SidebarGroupLabel>Logs</SidebarGroupLabel>
           <SidebarGroupContent>