feat(web): add error boundaries, 404 page, loading states, and SEO metadata

- Add branded not-found.tsx with navigation links
- Add global error.tsx boundary with retry and error digest display
- Add root loading.tsx skeleton for route transitions
- Expand root layout metadata: OpenGraph, Twitter cards, robots, viewport
- Add sitemap.ts and robots.ts for SEO
- Add search page and listing detail metadata via route layouts

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/app/loading.tsx

@@ -0,0 +1,38 @@
+export default function RootLoading() {
+  return (
+    <div className="flex min-h-screen flex-col bg-background">
+      {/* Header skeleton */}
+      <div className="border-b">
+        <div className="mx-auto flex h-16 max-w-7xl items-center justify-between px-4">
+          <div className="h-8 w-32 animate-pulse rounded bg-muted" />
+          <div className="flex gap-3">
+            <div className="h-8 w-20 animate-pulse rounded bg-muted" />
+            <div className="h-8 w-20 animate-pulse rounded bg-muted" />
+          </div>
+        </div>
+      </div>
+
+      {/* Content skeleton */}
+      <div className="mx-auto w-full max-w-7xl flex-1 px-4 py-8">
+        <div className="h-8 w-64 animate-pulse rounded bg-muted" />
+        <div className="mt-2 h-4 w-96 animate-pulse rounded bg-muted" />
+
+        <div className="mt-8 grid gap-4 sm:grid-cols-2 lg:grid-cols-3">
+          {Array.from({ length: 6 }).map((_, i) => (
+            <div key={i} className="rounded-lg border bg-card shadow-sm">
+              <div className="aspect-[16/10] animate-pulse rounded-t-lg bg-muted" />
+              <div className="p-4">
+                <div className="h-5 w-3/4 animate-pulse rounded bg-muted" />
+                <div className="mt-2 h-4 w-1/2 animate-pulse rounded bg-muted" />
+                <div className="mt-3 flex gap-2">
+                  <div className="h-6 w-16 animate-pulse rounded bg-muted" />
+                  <div className="h-6 w-16 animate-pulse rounded bg-muted" />
+                </div>
+              </div>
+            </div>
+          ))}
+        </div>
+      </div>
+    </div>
+  );
+}