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/robots.ts

@@ -0,0 +1,16 @@
+import type { MetadataRoute } from 'next';
+
+export default function robots(): MetadataRoute.Robots {
+  const siteUrl = process.env['NEXT_PUBLIC_SITE_URL'] || 'https://goodgo.vn';
+
+  return {
+    rules: [
+      {
+        userAgent: '*',
+        allow: '/',
+        disallow: ['/dashboard/', '/admin/', '/auth/', '/api/'],
+      },
+    ],
+    sitemap: `${siteUrl}/sitemap.xml`,
+  };
+}