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/robots.ts

@@ -0,0 +1,14 @@
+import { MetadataRoute } from 'next';
+
+export default function robots(): MetadataRoute.Robots {
+  const siteUrl = process.env.NEXT_PUBLIC_SITE_URL || 'http://localhost:3000';
+
+  return {
+    rules: {
+      userAgent: '*',
+      allow: '/',
+      disallow: ['/api/', '/_next/', '/admin/'],
+    },
+    sitemap: `${siteUrl}/sitemap.xml`,
+  };
+}