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/error.tsx

@@ -0,0 +1,62 @@
+'use client';
+
+import { useEffect } from 'react';
+
+export default function GlobalError({
+  error,
+  reset,
+}: {
+  error: Error & { digest?: string };
+  reset: () => void;
+}) {
+  useEffect(() => {
+    console.error('Unhandled error:', error);
+  }, [error]);
+
+  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="mx-auto flex h-16 w-16 items-center justify-center rounded-full bg-destructive/10">
+          <svg
+            className="h-8 w-8 text-destructive"
+            fill="none"
+            stroke="currentColor"
+            viewBox="0 0 24 24"
+          >
+            <path
+              strokeLinecap="round"
+              strokeLinejoin="round"
+              strokeWidth={2}
+              d="M12 9v2m0 4h.01m-6.938 4h13.856c1.54 0 2.502-1.667 1.732-2.5L13.732 4c-.77-.833-1.964-.833-2.732 0L4.082 16.5c-.77.833.192 2.5 1.732 2.5z"
+            />
+          </svg>
+        </div>
+        <h1 className="mt-4 text-2xl font-bold tracking-tight">
+          Đã xảy ra lỗi
+        </h1>
+        <p className="mt-2 text-muted-foreground">
+          Rất tiếc, đã có lỗi xảy ra. Vui lòng thử lại.
+        </p>
+        {error.digest && (
+          <p className="mt-1 text-xs text-muted-foreground">
+            Mã lỗi: {error.digest}
+          </p>
+        )}
+        <div className="mt-8 flex justify-center gap-3">
+          <button
+            onClick={reset}
+            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"
+          >
+            Thử lại
+          </button>
+          <a
+            href="/"
+            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"
+          >
+            Về trang chủ
+          </a>
+        </div>
+      </div>
+    </div>
+  );
+}