Demo: rate-limit and error handling UX #223

Closed as not planned along with the rest of EPIC #217: the initiative was abandoned after confirming that GitHub's /search/code endpoint doesn't include CORS headers on its actual responses (even on success), which cannot be worked around without a server-side relay — something explicitly out of scope. Notably, this is the exact failure mode this sub-issue tried to make resilient (rate-limit/network-error handling with retries) — the retries worked as designed but could never succeed against a response permanently missing the CORS header. See #217 for the full root-cause writeup. No code was merged; the local branch for this layer was discarded.