Dataset tab switches render instantly from a shared curated cache

[aperson30]

Why

Team/Overview -> Dataset tab switches paid a full network round trip + loading-spinner flash every time, even though the curated grid is deterministic. Two causes:

1. /api/search sends no Cache-Control, so the browser never reuses the request.
2. Homepage is a plain top-level route — React fully unmounts/remounts it on tab switch, so useDashboard's mount effect re-fetches from scratch every time.

The existing idle warm-up (prefetchCurated) already fetched the same curated data moments after app boot, but only used it to preload preview images and threw the JSON away — so it did nothing to prevent the refetch.

What changed

• helpers/curatedCache.ts (new): a module-scope cache that is the single source of truth for the curated (tumor/no-tumor, sort_by=quality) fetch. Both the app-boot warm-up and useDashboard's mount effect read/write it, de-duped via an in-flight promise.
• useDashboard: seeds previewIds/previewMetadata/loading from the cache via a lazy useState initializer when present, so a warm tab switch renders the grid on the very first paint — no skeleton, no request. Cold loads (first visit before warm-up settles, or a genuine fetch failure) fall back to the previous fetch-and-wait behavior unchanged. A total-failure response is not cached, so a transient network blip doesn't wedge the grid empty for the rest of the session.
• App.tsx: swapped the old prefetchCurated import for warmCuratedCache, same idle-callback timing.
• Removed helpers/prefetchCurated.ts (superseded).

Filtered/deep-linked searches are unaffected — only the no-filter curated view is cacheable.

Verified

• npm run typecheck and npm run build both clean (run in a scratch clone on bdmap1, not the prod checkout).