Enhance search engine optimization

Comprehensive SEO improvements to boost search visibility and social sharing:

Meta Tags & Structured Data:
- Add JSON-LD structured data (Schema.org WebApplication)
- Enhance meta description with actionable keywords
- Expand keywords for better search coverage
- Add robots meta tag with max-snippet directives
- Add generator meta tag
- Add Twitter card type (summary_large_image)
- Add Open Graph image placeholder with alt text
- Update all titles and descriptions consistently

Mobile & Theme Optimization:
- Add theme-color meta tags for light/dark modes
- Add mobile-web-app-capable meta tags
- Add Apple-specific mobile meta tags
- Improve mobile browser integration

Performance:
- Add preload hints for critical CSS and JS resources
- Add dns-prefetch for CDN resources
- Add crossorigin attribute to preconnect hints

Crawler Configuration:
- Create robots.txt with sitemap reference
- Create sitemap.xml with main URLs
- Allow all crawlers with proper directives

These enhancements improve discoverability, social sharing, mobile experience,
and overall search engine ranking potential.

sitemap.xml

@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
+        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9
+        http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd">
+  <!-- Main Page -->
+  <url>
+    <loc>https://metajs.xi-xu.me/</loc>
+    <lastmod>2025-11-12</lastmod>
+    <changefreq>weekly</changefreq>
+    <priority>1.0</priority>
+  </url>
+
+  <!-- GitHub Repository -->
+  <url>
+    <loc>https://github.com/xixu-me/META.js-Customizer</loc>
+    <lastmod>2025-11-12</lastmod>
+    <changefreq>weekly</changefreq>
+    <priority>0.8</priority>
+  </url>
+</urlset>