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

src/pages/about.astro

@@ -23,7 +23,7 @@ const prefix = isZh ? '/zh' : '';
 <Layout title={isZh ? '关于' : 'About'}>
   <GlassHeader lang={lang} client:load transition:persist="header" />
 
-  <main class="min-h-screen pt-24 pb-20">
+  <main id="main-content" class="min-h-screen pt-24 pb-20">
     <Container>
       <section class="page-content-main" id="overview">
         <div class="mb-6 flex flex-wrap gap-2 text-sm">