chore(ui): use proxy server for backend API calls; simplified k8s deployment (#2350)

# What does this PR do?
- no more CORS middleware needed


## Test Plan
### Local test
llama stack run starter --image-type conda
npm run dev
verify UI works in browser

### Deploy to k8s
temporarily change ui-k8s.yaml.template to load from PR commit
<img width="604" alt="image"
src="https://github.com/user-attachments/assets/87fa2e52-1e93-4e32-9e0f-5b283b7a37b3"
/>

sh ./apply.sh
$ kubectl get services
go to external_ip:8322 and play around with UI
<img width="1690" alt="image"
src="https://github.com/user-attachments/assets/5b7ec827-4302-4435-a9eb-df423676d873"
/>

llama_stack/ui/lib/client.ts

@@ -1,5 +1,6 @@
 import LlamaStackClient from "llama-stack-client";
 
 export const client = new LlamaStackClient({
-  baseURL: process.env.NEXT_PUBLIC_LLAMA_STACK_BASE_URL,
+  baseURL:
+    typeof window !== "undefined" ? `${window.location.origin}/api` : "/api",
 });