Add curated essays and reading paths

Introduce a Library page and wire it into the main nav.
Add blog frontmatter fields: category, featuredEssay (default false),
and readingOrder (positive integer). Update several posts to mark
featured essays and assign readingOrder for the recommended path.

src/content/blog/hello-world.md

@@ -3,6 +3,9 @@ title: "Welcome to the Den"
 description: "First proper post. Why I built this site, what it runs on, and what to expect."
 pubDate: 2026-03-01
 tags: ["self-hosting", "privacy", "personal-web", "infrastructure"]
+category: "building"
+featuredEssay: true
+readingOrder: 1
 draft: false
 ---