perf(storefront): stop paying a Worker round trip for server islands on every static page view#291

What happened

Prerendered pages still fire two uncached server-island fetches per view: _server-islands/UserProfile and _server-islands/SidebarUserSection.

• Both respond with cache-control: no-store, cf-cache-status: BYPASS
• Measured TTFB from UB: 0.8s cold-ish, up to 1.6s under load on one run
• That is two Worker invocations per page view for data that only matters to signed-in users

They do not block paint, so impact is moderate, but anonymous visitors pay this on every single navigation.

Plan

Either:

1. Return a short-TTL private cache response for the anonymous case so repeat views hit the browser cache, or
2. Replace the island with a tiny client-side session probe and render the sidebar shell statically.

Acceptance criteria

• Anonymous repeat page views make zero server-island Worker invocations
• Signed-in state still renders correctly after client reconciliation