fix(docs/search): handle search index loading and failure states (#3184)

* fix(docs/search): show loading state and replay query when index isn't ready

Previously performSearch rendered "No results found" while the
MiniSearch index was still loading, which is misleading on slow
networks. Track the last query as pendingQuery and replay it from
fetchSearchIndex once the index is ready; render a distinct
"Failed to load search index" message on fetch error so the
loading message doesn't persist indefinitely.

Fixes #3180.

* fix(docs/search): close stale-replay race and consolidate status messages

Drop pendingQuery synchronously in the input handler when the query
falls below the minimum length, so a fetchSearchIndex that resolves
within the 300ms debounce window after the user clears the field can
no longer replay the cleared query into an empty input.

Move the idle, loading, and failed copy into a single
renderSearchStatus(kind) helper so the four .search-no-results
renderings can only drift in one place.