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

components/shopify/search-dialog.tsx

@@ -86,8 +86,9 @@ const SearchDialog: React.FC = () => {
         variant="ghost"
         size="icon"
         aria-label="Search"
+        className="rounded-full"
       >
-        <RiSearchLine size={20} />
+        <RiSearchLine className="size-5" />
       </Button>
 
       <CommandDialog open={open} onOpenChange={(next) => !next && close()}>