perf(marketplace): lazy-mount grid cards + open the detail page on click

- LazyMount: mount a grid card only when it scrolls within 600px (skeleton-space
  placeholder otherwise), so a long feed no longer mounts every card + its
  hooks/DOM up front. Falls back to immediate render without IntersectionObserver.
- GearSwapItemCard: clicking the card now opens /listing/:pubkey/:identifier
  (was opening a DM). 'Contact Seller' still opens the DM. Card set to h-full so
  the lazy wrapper keeps the equal-height grid.

Gate: 101 tests, 0 errors/warnings, boot smoke green.

src/components/GearSwapItemCard.tsx

@@ -127,12 +127,12 @@ export function GearSwapItemCard({ item, trustScore, salesCount }: GearSwapItemC
 
   return (
     <Card
-      className={`bg-black/90 border-2 group cursor-pointer transition-all hover:scale-[1.02] ${
+      className={`bg-black/90 border-2 group cursor-pointer transition-all hover:scale-[1.02] h-full ${
         isWTB
           ? 'border-blue-500/20 hover:border-blue-500/60 hover:shadow-[0_0_30px_rgba(59,130,246,0.2)]'
           : 'border-orange-500/20 hover:border-orange-500/60 hover:shadow-[0_0_30px_rgba(255,102,0,0.2)]'
       }`}
-      onClick={() => navigate(`/messages/${npub}`)}
+      onClick={() => navigate(`/listing/${item.pubkey}/${item.identifier}`)}
     >
       <CardContent className="p-0">
         {/* Image */}

src/components/LazyMount.tsx

@@ -0,0 +1,60 @@
+import { useEffect, useRef, useState, type ReactNode } from 'react';
+
+/**
+ * Defer mounting `children` until the wrapper scrolls near the viewport.
+ *
+ * Long grids (e.g. the marketplace) otherwise mount every card — and every
+ * hook/query/DOM node each one pulls in — up front. With this, off-screen cards
+ * stay as a cheap placeholder until they're ~600px away, then mount once and
+ * stay mounted. Falls back to immediate render when IntersectionObserver is
+ * unavailable.
+ */
+export function LazyMount({
+  children,
+  minHeight = 300,
+  rootMargin = '600px',
+  className,
+}: {
+  children: ReactNode;
+  /** Reserve height so the page doesn't jump when the card mounts. */
+  minHeight?: number;
+  rootMargin?: string;
+  className?: string;
+}) {
+  const ref = useRef<HTMLDivElement>(null);
+  const [visible, setVisible] = useState(false);
+
+  useEffect(() => {
+    if (visible) return;
+    const el = ref.current;
+    if (!el) return;
+
+    if (typeof IntersectionObserver === 'undefined') {
+      setVisible(true);
+      return;
+    }
+
+    const observer = new IntersectionObserver(
+      (entries) => {
+        if (entries.some((e) => e.isIntersecting)) {
+          setVisible(true);
+          observer.disconnect();
+        }
+      },
+      { rootMargin },
+    );
+    observer.observe(el);
+    return () => observer.disconnect();
+  }, [visible, rootMargin]);
+
+  return (
+    <div
+      ref={ref}
+      className={className}
+      style={visible ? undefined : { minHeight }}
+      aria-busy={!visible || undefined}
+    >
+      {visible ? children : null}
+    </div>
+  );
+}

src/pages/GearSwapPage.tsx

@@ -13,6 +13,7 @@ import { DropdownMenu, DropdownMenuContent, DropdownMenuCheckboxItem, DropdownMe
 import { useMarketplaceItems } from '@/hooks/useMarketplace';
 import { useCurrentUser } from '@/hooks/useCurrentUser';
 import { useBatchTrustScores } from '@/hooks/useBatchTrustScores';
+import { LazyMount } from '@/components/LazyMount';
 import { useBatchSellerSales } from '@/hooks/useSellerSales';
 import { useFollowList } from '@/hooks/useFollowList';
 import { GearSwapItemCard } from '@/components/GearSwapItemCard';
@@ -410,13 +411,14 @@ export default function GearSwapPage() {
         {!isLoading && !error && filteredItems.length > 0 && (
           <div className="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 xl:grid-cols-4 gap-4">
             {filteredItems.map((item) => (
-              <GearSwapItemCard
-                key={item.id}
-                item={item}
-                trustScore={trustScores?.get(item.pubkey)}
-                salesCount={sellerSales?.get(item.pubkey) ?? 0}
-                isFollowing={followingSet.has(item.pubkey)}
-              />
+              <LazyMount key={item.id} className="h-full">
+                <GearSwapItemCard
+                  item={item}
+                  trustScore={trustScores?.get(item.pubkey)}
+                  salesCount={sellerSales?.get(item.pubkey) ?? 0}
+                  isFollowing={followingSet.has(item.pubkey)}
+                />
+              </LazyMount>
             ))}
           </div>
         )}