Add a Shop menu that lists the store's collections

The header now carries a "Shop" dropdown that fetches the collection list
the first time it's opened and links each entry to its collection page.

Fetching is deferred so the request only happens on open: useCollections
loads on mount, which would put a collections query on every page. The new
useCollectionsOnDemand holds off until load() runs, keeps the result for
later opens, and clears its guard on failure so a retry can go through.

Rendered in the desktop nav as a floating panel and in the mobile menu as
an inline section, following the hand-rolled pattern in account-menu.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D1vsTA4MX6qFd7nqSAKtdV

hooks/use-shopify-collections.ts

@@ -1,6 +1,6 @@
 'use client';
 
-import { useState, useEffect, useCallback } from 'react';
+import { useState, useEffect, useCallback, useRef } from 'react';
 import {
   getCollections,
   getCollectionProducts,
@@ -60,6 +60,36 @@ export function useCollections(first = 50) {
   return { collections, loading, error, refetch: fetchCollections };
 }
 
+// Deferred variant of useCollections: nothing is requested until `load` runs,
+// so a menu can hold off until it's actually opened. The fetch happens once —
+// re-opening reuses what's already in state.
+export function useCollectionsOnDemand(first = 50) {
+  const [collections, setCollections] = useState<Collection[]>([]);
+  const [loading, setLoading] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+  const requested = useRef(false);
+
+  const load = useCallback(async () => {
+    if (requested.current) return;
+    requested.current = true;
+
+    try {
+      setLoading(true);
+      setError(null);
+      setCollections(await getCollections(first));
+    } catch (err) {
+      console.error('Error fetching collections:', err);
+      // Let the next open (or a retry) try again.
+      requested.current = false;
+      setError(err instanceof Error ? err.message : 'Failed to load collections');
+    } finally {
+      setLoading(false);
+    }
+  }, [first]);
+
+  return { collections, loading, error, load };
+}
+
 // Hook for fetching products in a collection
 export function useCollectionProducts(
   handle: string | null,