improve lighthouse scores: image priority and long-poll removal

- Add priority attr to product_card, thread through to responsive_image
- First 2 featured products get fetchpriority="high" and eager loading
- Remove longPollFallbackMs to avoid unnecessary fallback attempt

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/js/app.js

@@ -286,7 +286,6 @@ const Lightbox = {
 
 const csrfToken = document.querySelector("meta[name='csrf-token']").getAttribute("content")
 const liveSocket = new LiveSocket("/live", Socket, {
-  longPollFallbackMs: 2500,
   params: {_csrf_token: csrfToken},
   hooks: {...colocatedHooks, ColorSync, Lightbox, CartPersist, CartDrawer},
 })