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/sitemap.ts

@@ -0,0 +1,68 @@
+import { MetadataRoute } from 'next';
+import { allPosts, allPages } from 'contentlayer2/generated';
+
+export default function sitemap(): MetadataRoute.Sitemap {
+  const siteUrl = process.env.NEXT_PUBLIC_SITE_URL || 'http://localhost:3000';
+
+  // Homepage
+  const homepage = {
+    url: siteUrl,
+    lastModified: new Date(),
+    changeFrequency: 'daily' as const,
+    priority: 1,
+  };
+
+  // Blog listing page
+  const blogPage = {
+    url: `${siteUrl}/blog`,
+    lastModified: new Date(),
+    changeFrequency: 'daily' as const,
+    priority: 0.9,
+  };
+
+  // Tags page
+  const tagsPage = {
+    url: `${siteUrl}/tags`,
+    lastModified: new Date(),
+    changeFrequency: 'weekly' as const,
+    priority: 0.7,
+  };
+
+  // All blog posts
+  const posts = allPosts
+    .filter((post) => post.status === 'published')
+    .map((post) => ({
+      url: `${siteUrl}${post.url}`,
+      lastModified: new Date(post.updated_at || post.published_at || post.created_at || Date.now()),
+      changeFrequency: 'weekly' as const,
+      priority: 0.8,
+    }));
+
+  // All pages
+  const pages = allPages
+    .filter((page) => page.status === 'published')
+    .map((page) => ({
+      url: `${siteUrl}${page.url}`,
+      lastModified: new Date(page.updated_at || page.published_at || page.created_at || Date.now()),
+      changeFrequency: 'monthly' as const,
+      priority: 0.6,
+    }));
+
+  // All unique tags
+  const allTags = Array.from(
+    new Set(
+      allPosts
+        .filter((post) => post.status === 'published' && post.tags)
+        .flatMap((post) => post.tags || [])
+    )
+  );
+
+  const tagPages = allTags.map((tag) => ({
+    url: `${siteUrl}/tags/${encodeURIComponent(tag.toLowerCase().replace(/\s+/g, '-'))}`,
+    lastModified: new Date(),
+    changeFrequency: 'weekly' as const,
+    priority: 0.5,
+  }));
+
+  return [homepage, blogPage, tagsPage, ...posts, ...pages, ...tagPages];
+}