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"
/>

docs/source/distributions/k8s/apply.sh

@@ -13,8 +13,6 @@ export POSTGRES_PASSWORD=${POSTGRES_PASSWORD:-llamastack}
 export INFERENCE_MODEL=${INFERENCE_MODEL:-meta-llama/Llama-3.2-3B-Instruct}
 export SAFETY_MODEL=${SAFETY_MODEL:-meta-llama/Llama-Guard-3-1B}
 
-export NEXT_PUBLIC_LLAMA_STACK_BASE_URL=${NEXT_PUBLIC_LLAMA_STACK_BASE_URL:-}
-
 set -euo pipefail
 set -x
 

docs/source/distributions/k8s/ui-k8s.yaml.template

@@ -22,8 +22,8 @@ spec:
         image: node:18-alpine
         command: ["/bin/sh"]
         env:
-        - name: NEXT_PUBLIC_LLAMA_STACK_BASE_URL
-          value: ${NEXT_PUBLIC_LLAMA_STACK_BASE_URL}
+        - name: LLAMA_STACK_BACKEND_URL
+          value: "http://llama-stack-service:8321"
         - name: LLAMA_STACK_UI_PORT
           value: "8322"
         args: