docs: replace error-boundary-retry screenshot with the actual error state

The previous capture froze on the "Loaded." success state, which doesn't
illustrate what the page is documenting. The new capture shows the
"Couldn't load. / Service unavailable / Retry" UI that demonstrates the
ErrorBoundary's retry affordance.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: codemonkeychris