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>

package.json

@@ -33,8 +33,10 @@
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
     "rehype-autolink-headings": "^7.1.0",
+    "rehype-pretty-code": "^0.14.1",
     "rehype-slug": "^6.0.0",
     "remark-gfm": "^4.0.1",
+    "shiki": "^3.15.0",
     "tailwind-merge": "^3.4.0",
     "unist-util-visit": "^5.0.0"
   },