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/not-found.tsx

@@ -0,0 +1,31 @@
+import Link from 'next/link';
+
+export default function NotFound() {
+  return (
+    <div className="flex min-h-screen flex-col items-center justify-center bg-background px-4">
+      <div className="mx-auto max-w-md text-center">
+        <div className="text-8xl font-bold text-primary/20">404</div>
+        <h1 className="mt-4 text-2xl font-bold tracking-tight">
+          Không tìm thấy trang
+        </h1>
+        <p className="mt-2 text-muted-foreground">
+          Trang bạn đang tìm không tồn tại hoặc đã được di chuyển.
+        </p>
+        <div className="mt-8 flex justify-center gap-3">
+          <Link
+            href="/"
+            className="inline-flex h-10 items-center justify-center rounded-md bg-primary px-6 text-sm font-medium text-primary-foreground shadow transition-colors hover:bg-primary/90"
+          >
+            Về trang chủ
+          </Link>
+          <Link
+            href="/search"
+            className="inline-flex h-10 items-center justify-center rounded-md border border-input bg-background px-6 text-sm font-medium shadow-sm transition-colors hover:bg-accent hover:text-accent-foreground"
+          >
+            Tìm kiếm
+          </Link>
+        </div>
+      </div>
+    </div>
+  );
+}