feat: CMS-editable nav, categories taxonomy, site settings

- Move top nav from hugo.toml menus to data/nav.yml (CMS-editable)
- Add categories taxonomy alongside tags
- Add Decap CMS settings collection for nav management
- Add categories field to articles and essays in CMS config

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

site/layouts/partials/nav.html

@@ -1,8 +1,8 @@
 <nav>
   <ul><li><a href="/"><strong>{{ .Site.Title }}</strong></a></li></ul>
   <ul>
-    {{- range .Site.Menus.main }}
-    <li><a href="{{ .URL }}"{{ if $.IsMenuCurrent "main" . }} aria-current="page"{{ end }}>{{ .Name }}</a></li>
+    {{- range sort .Site.Data.nav "weight" }}
+    <li><a href="{{ .url }}"{{ if eq (relURL .url) $.RelPermalink }} aria-current="page"{{ end }}>{{ .name }}</a></li>
     {{- end }}
   </ul>
 </nav>