Polish full-stack evaluation demo flow

[fengting124]

Summary

• add a small frontend test boundary for API error formatting
• map Vite/backend gateway failures to a clear backend availability message
• document the full-stack evaluation demo path without requiring model weights

Why

The evaluation execution framework and admin workbench are now connected, but the demo experience still needed a clearer operator path. This keeps the current UI style and avoids fake frontend data while making offline-backend failures readable.

Verification

• npm run test
• npm run lint
• npm run build
• mvn -B test
• git diff --check

Deferred

• Full interactive Docker demo remains deferred until Docker is available locally.
• Real model weights remain deferred until a GPU server is prepared.