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

hooks/use-shopify-collections.ts

@@ -26,10 +26,40 @@ export interface CollectionWithProducts extends Collection {
   products: Product[];
 }
 
+export type CollectionSortKey =
+  | 'COLLECTION_DEFAULT'
+  | 'BEST_SELLING'
+  | 'CREATED'
+  | 'PRICE'
+  | 'TITLE';
+
 interface UseCollectionProductsOptions {
   first?: number;
-  sortKey?: 'BEST_SELLING' | 'CREATED' | 'PRICE' | 'TITLE';
+  after?: string | null;
+  sortKey?: CollectionSortKey;
   reverse?: boolean;
+  /** Raw `input` strings from the connection's `filters` facets. */
+  filterInputs?: string[];
+}
+
+export interface CollectionProductsPage {
+  collection: Collection | null;
+  products: Product[];
+  filters: ProductFilterFacet[];
+  hasNextPage: boolean;
+  endCursor: string | null;
+}
+
+export interface ProductFilterFacet {
+  id: string;
+  label: string;
+  type: 'LIST' | 'PRICE_RANGE' | 'BOOLEAN';
+  values: Array<{
+    id: string;
+    label: string;
+    count: number;
+    input: string;
+  }>;
 }
 
 // Fetch all collections
@@ -45,19 +75,65 @@ export async function getCollections(first = 50): Promise<Collection[]> {
 // Fetch products in a collection by handle
 export async function getCollectionProducts(
   handle: string,
-  { first = 50, sortKey = 'BEST_SELLING', reverse = false }: UseCollectionProductsOptions = {}
+  options: UseCollectionProductsOptions = {}
 ): Promise<CollectionWithProducts | null> {
+  const page = await getCollectionProductsPage(handle, options);
+  if (!page.collection) return null;
+
+  return { ...page.collection, products: page.products };
+}
+
+// Same fetch, but keeps the cursor and facet list for filtering and paging.
+export async function getCollectionProductsPage(
+  handle: string,
+  {
+    first = 50,
+    after = null,
+    sortKey = 'COLLECTION_DEFAULT',
+    reverse = false,
+    filterInputs = [],
+  }: UseCollectionProductsOptions = {}
+): Promise<CollectionProductsPage> {
+  const filters = filterInputs.flatMap((input) => {
+    try {
+      return [JSON.parse(input)];
+    } catch {
+      console.warn('Ignoring malformed product filter input:', input);
+      return [];
+    }
+  });
+
   const response = await shopifyFetch({
     query: GET_COLLECTION_PRODUCTS_QUERY,
-    variables: { handle, first, sortKey, reverse },
+    variables: {
+      handle,
+      first,
+      after,
+      sortKey,
+      reverse,
+      filters: filters.length ? filters : null,
+    },
   });
 
   const collection = response.data.collection;
-  if (!collection) return null;
+  if (!collection) {
+    return {
+      collection: null,
+      products: [],
+      filters: [],
+      hasNextPage: false,
+      endCursor: null,
+    };
+  }
+
+  const { edges, pageInfo, filters: facets } = collection.products;
 
   return {
-    ...collection,
-    products: collection.products.edges.map((edge: { node: Product }) => edge.node),
+    collection,
+    products: edges.map((edge: { node: Product }) => edge.node),
+    filters: facets ?? [],
+    hasNextPage: Boolean(pageInfo?.hasNextPage),
+    endCursor: pageInfo?.endCursor ?? null,
   };
 }