perf(storefront): make the homepage hero paint immediately#287

What happened

Lighthouse 13 (mobile, Slow 4G, 4x CPU) scores the homepage at 53 while product pages score 92+. LCP is the hero carousel image at 6.2s, and the network is not the problem:

• TTFB 158ms, hero image download 158ms
• 1,438ms element render delay waiting on the main thread
• Main thread during load: 1,517ms style & layout, 1,334ms script evaluation
• Long tasks: client.D6_eju8K.js 429ms, PostHog array.js 199ms, inline doc script 182ms
• The active hero <img> has no fetchpriority="high" (Lighthouse lcp-discovery checklist flags this)
• The hero also fires twice at different widths (480w then 360w at 1,361ms), suggesting a srcset/DPR mismatch

12 card-add-button islands hydrate eagerly via client="load" even though most sit below the fold.

What I expect

The hero should be the first thing painted, not the last.

Plan

1. Add fetchpriority="high" to the active hero slide img and <link rel="preload" as="image"> it.
2. Check why a second 360w variant is requested late; align srcset/sizes with the actual rendered slot.
3. Switch below-fold islands (card-add-button, brand strip bits) to client="visible".
4. Re-run Lighthouse mobile on /.

Acceptance criteria

• Homepage mobile performance score >= 90
• Homepage LCP < 2.5s under Lighthouse mobile throttling
• CLS stays at 0
• Hero image has exactly one network request at the correct width