Add RSS feed, sitemap, robots.txt, and code syntax highlighting

Implements essential blog features:

1. RSS Feed (/feed.xml)
   - Latest 20 posts with full content
   - Proper XML escaping and CDATA sections
   - Includes tags, authors, and descriptions
   - Auto-discovery link in HTML head

2. Sitemap (/sitemap.xml)
   - All posts, pages, and tag pages
   - Proper lastModified dates and priorities
   - Automatic generation via Next.js built-in support

3. Robots.txt (/robots.txt)
   - Allow all crawlers
   - Disallow API and admin routes
   - Links to sitemap for better SEO

4. Code Syntax Highlighting
   - Using rehype-pretty-code + Shiki
   - GitHub Dark/Light themes based on user preference
   - Line numbers for all code blocks
   - Support for highlighted lines
   - Inline code styling
   - Code title support

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/feed.xml/route.ts

@@ -0,0 +1,63 @@
+import { allPosts } from 'contentlayer2/generated';
+import { siteConfig } from '@/lib/config';
+
+export async function GET() {
+  const sortedPosts = allPosts
+    .filter((post) => post.status === 'published')
+    .sort((a, b) => {
+      const dateA = a.published_at ? new Date(a.published_at).getTime() : 0;
+      const dateB = b.published_at ? new Date(b.published_at).getTime() : 0;
+      return dateB - dateA;
+    })
+    .slice(0, 20); // Latest 20 posts
+
+  const siteUrl = process.env.NEXT_PUBLIC_SITE_URL || 'http://localhost:3000';
+
+  const rss = `<?xml version="1.0" encoding="UTF-8"?>
+<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom" xmlns:content="http://purl.org/rss/1.0/modules/content/">
+  <channel>
+    <title>${escapeXml(siteConfig.name)}</title>
+    <link>${siteUrl}</link>
+    <description>${escapeXml(siteConfig.description)}</description>
+    <language>${siteConfig.defaultLocale.replace('_', '-')}</language>
+    <lastBuildDate>${new Date().toUTCString()}</lastBuildDate>
+    <atom:link href="${siteUrl}/feed.xml" rel="self" type="application/rss+xml"/>
+    ${sortedPosts
+      .map((post) => {
+        const postUrl = `${siteUrl}${post.url}`;
+        const pubDate = post.published_at
+          ? new Date(post.published_at).toUTCString()
+          : new Date(post.created_at || Date.now()).toUTCString();
+
+        return `
+    <item>
+      <title>${escapeXml(post.title)}</title>
+      <link>${postUrl}</link>
+      <guid isPermaLink="true">${postUrl}</guid>
+      <description>${escapeXml(post.description || post.custom_excerpt || post.title)}</description>
+      ${post.body?.html ? `<content:encoded><![CDATA[${post.body.html}]]></content:encoded>` : ''}
+      <pubDate>${pubDate}</pubDate>
+      ${post.authors?.map((author) => `<author>${escapeXml(author)}</author>`).join('\n      ') || ''}
+      ${post.tags?.map((tag) => `<category>${escapeXml(tag)}</category>`).join('\n      ') || ''}
+    </item>`;
+      })
+      .join('')}
+  </channel>
+</rss>`;
+
+  return new Response(rss, {
+    headers: {
+      'Content-Type': 'application/xml; charset=utf-8',
+      'Cache-Control': 'public, max-age=3600, s-maxage=3600',
+    },
+  });
+}
+
+function escapeXml(unsafe: string): string {
+  return unsafe
+    .replace(/&/g, '&amp;')
+    .replace(/</g, '&lt;')
+    .replace(/>/g, '&gt;')
+    .replace(/"/g, '&quot;')
+    .replace(/'/g, '&apos;');
+}