#186 fix(demo): keep errors prominent until recovery

Summary

Makes demo failures harder to miss and keeps them visible until a later operation succeeds.

• keeps the current error banner visible while a retry is in progress
• clears the banner only after a successful operation
• makes the banner sticky and more visually prominent

Validation

• node --check jev_ultrafast/static/app.js
• uv run ruff check .
• uv run pytest — 31 passed
• git diff --check

Addresses the persistent-error-banner portion of #159.

---

Summary by cubic

Keeps the demo error banner visible during retries and only clears it after a successful operation, and makes it sticky and more visually prominent so failures are harder to miss. Addresses the persistent-error-banner portion of #159.