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>

contentlayer.config.ts

@@ -3,6 +3,7 @@ import { visit } from 'unist-util-visit';
 import rehypeSlug from 'rehype-slug';
 import rehypeAutolinkHeadings from 'rehype-autolink-headings';
 import remarkGfm from 'remark-gfm';
+import rehypePrettyCode from 'rehype-pretty-code';
 
 export const Post = defineDocumentType(() => ({
   name: 'Post',
@@ -86,6 +87,16 @@ export default makeSource({
   markdown: {
     remarkPlugins: [remarkGfm],
     rehypePlugins: [
+      [
+        rehypePrettyCode,
+        {
+          theme: {
+            dark: 'github-dark',
+            light: 'github-light',
+          },
+          keepBackground: false,
+        },
+      ],
       rehypeSlug,
       [rehypeAutolinkHeadings, { behavior: 'wrap' }],
       /**