Perceived-latency pass: loading skeletons, nav prefetch, auth-path write, /chestshop cache

Post-PAR-129/130 the worst pages load, but the app still feels sluggish. Causes are on the common/perceived path, not individual page queries:

1. No loading.tsx anywhere + force-dynamic + prefetch=false → every click is a cold blocking server round-trip with no feedback. Add route-level loading skeletons; enable prefetch on the primary header nav only (keep content links cold, per anti-scraping stance).
2. getViewer writes on every authenticated page — upsertIdentity runs before the identity read on every render. Reorder to read-first, upsert only when the link is missing or the name changed. Saves a write round-trip to the (remote, ~50ms RTT) Bloom.host DB on nearly every page. Faro floor: /login (no DB) ~138ms vs /link (auth) ~317ms.
3. /chestshop ~4s — listItems/countItems do a full-table GROUP BY item_key over all chestshop_sale every load. Cache the no-search default page (like /market in PAR-130).

UX scope chosen by user: skeletons + prefetch on main nav.