Add manifest, robots.txt, sitemap.xml, and thumbnail image for improved SEO and site structure

public/sitemap.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  <url>
+    <loc>https://untrace.realbrain.cc/untrace/</loc>
+    <lastmod>2025-06-23</lastmod>
+    <changefreq>weekly</changefreq>
+    <priority>1.0</priority>
+  </url>
+</urlset>