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/responses/items/function-call-item.tsx

@@ -1,7 +1,7 @@
 import {
   MessageBlock,
   ToolCallBlock,
-} from "@/components/ui/message-components";
+} from "@/components/chat-playground/message-components";
 import { FunctionCallItem } from "../utils/item-types";
 
 interface FunctionCallItemProps {

llama_stack/ui/components/responses/items/generic-item.tsx

@@ -1,7 +1,7 @@
 import {
   MessageBlock,
   ToolCallBlock,
-} from "@/components/ui/message-components";
+} from "@/components/chat-playground/message-components";
 import { BaseItem } from "../utils/item-types";
 
 interface GenericItemProps {

llama_stack/ui/components/responses/items/grouped-function-call-item.tsx

@@ -1,7 +1,7 @@
 import {
   MessageBlock,
   ToolCallBlock,
-} from "@/components/ui/message-components";
+} from "@/components/chat-playground/message-components";
 import { FunctionCallItem, FunctionCallOutputItem } from "../utils/item-types";
 
 interface GroupedFunctionCallItemProps {

llama_stack/ui/components/responses/items/message-item.tsx

@@ -1,4 +1,4 @@
-import { MessageBlock } from "@/components/ui/message-components";
+import { MessageBlock } from "@/components/chat-playground/message-components";
 import { MessageItem } from "../utils/item-types";
 
 interface MessageItemProps {

llama_stack/ui/components/responses/items/web-search-item.tsx

@@ -1,7 +1,7 @@
 import {
   MessageBlock,
   ToolCallBlock,
-} from "@/components/ui/message-components";
+} from "@/components/chat-playground/message-components";
 import { WebSearchCallItem } from "../utils/item-types";
 
 interface WebSearchItemProps {