feat: add SEO infrastructure, 404 page, accessibility and performance optimizations

- Add robots.txt for search engine crawling
- Enhance Layout.astro with complete SEO meta tags (OG, Twitter Card, canonical)
- Create custom 404 page with bilingual support
- Add skip link for accessibility
- Add main-content id to all major pages for keyboard navigation
- Add lazy loading to blog list and author card images

public/robots.txt

@@ -0,0 +1,13 @@
+# robots.txt for zhaoguiyang.com
+# https://zhaoguiyang.com
+
+User-agent: *
+Allow: /
+
+# Sitemap
+Sitemap: https://zhaoguiyang.com/sitemap-index.xml
+
+# Disallow admin/private areas (if any)
+Disallow: /api/
+Disallow: /_astro/
+Disallow: /assets/