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/sitemap.ts

@@ -0,0 +1,32 @@
+import type { MetadataRoute } from 'next';
+
+export default function sitemap(): MetadataRoute.Sitemap {
+  const siteUrl = process.env['NEXT_PUBLIC_SITE_URL'] || 'https://goodgo.vn';
+
+  return [
+    {
+      url: siteUrl,
+      lastModified: new Date(),
+      changeFrequency: 'daily',
+      priority: 1,
+    },
+    {
+      url: `${siteUrl}/search`,
+      lastModified: new Date(),
+      changeFrequency: 'daily',
+      priority: 0.9,
+    },
+    {
+      url: `${siteUrl}/login`,
+      lastModified: new Date(),
+      changeFrequency: 'monthly',
+      priority: 0.3,
+    },
+    {
+      url: `${siteUrl}/register`,
+      lastModified: new Date(),
+      changeFrequency: 'monthly',
+      priority: 0.3,
+    },
+  ];
+}