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/data/nav.yml

@@ -0,0 +1,23 @@
+- name: Home
+  url: /
+  weight: 1
+
+- name: Docs
+  url: /docs/
+  weight: 2
+
+- name: Notes
+  url: /notes/
+  weight: 3
+
+- name: Essays
+  url: /essays/
+  weight: 4
+
+- name: Articles
+  url: /articles/
+  weight: 5
+
+- name: Tags
+  url: /tags/
+  weight: 6