Add collection filters and sort, out-of-stock options, gallery drag

- Collections: filters and sort via the Storefront API — the products
  connection now takes `filters`/`after` and returns its facets, with
  cursor-based load more on the collection page
- Extract ProductFilters (renamed from SearchFilters) and a shared
  ProductToolbar so search and collections use the same chrome
- PDP: mark option values with no in-stock variant using a diagonal
  strike, computed against the other selected options
- PDP: pointer drag-scrolling for the mobile image carousel, since a
  scroll container doesn't drag with a mouse
- Header/cart: circular icon buttons, tighter spacing, and icon sizes
  set by class (Button's base CSS clamps un-classed svgs to size-4)

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jWbNNJLksC1QG8z8845FX

app/globals.css

@@ -94,6 +94,17 @@ h2,
   display: none;
 }
 
+/* Diagonal strike marking an unavailable product option (size pill, swatch) */
+.option-unavailable {
+  background-image: linear-gradient(
+    to top right,
+    transparent calc(50% - 0.5px),
+    currentColor calc(50% - 0.5px),
+    currentColor calc(50% + 0.5px),
+    transparent calc(50% + 0.5px)
+  );
+}
+
 /* Product description (HTML returned by the Storefront API) */
 .product-description p {
   margin-bottom: 1rem;