Add category taxonomy and navigation integration

Implements Issue #35 with category snippets, article category routing, category-aware templates, and category RSS feeds with tests.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

templates/components/nav.html

@@ -1,5 +1,6 @@
 {% load static core_tags %}
 {% get_nav_items "header" as header_items %}
+{% get_categories_nav as category_nav_items %}
 <nav class="sticky top-0 z-50 backdrop-blur-md bg-brand-light/80 dark:bg-brand-dark/80 border-b border-zinc-200 dark:border-zinc-800 transition-colors">
   <div class="max-w-7xl mx-auto px-6 h-20 flex items-center justify-between">
     <!-- Logo -->
@@ -15,6 +16,9 @@
       {% for item in header_items %}
         <a href="{{ item.url }}" class="hover:text-brand-cyan transition-colors"{% if item.open_in_new_tab %} target="_blank" rel="noopener noreferrer"{% endif %}>{{ item.title }}</a>
       {% endfor %}
+      {% for category in category_nav_items %}
+        <a href="{{ category.url }}" class="hover:text-brand-cyan transition-colors {% if category.url in request.path %}text-brand-cyan{% endif %}" {% if category.url in request.path %}aria-current="page"{% endif %}>{{ category.name }}</a>
+      {% endfor %}
       <a href="#newsletter" class="px-5 py-2.5 bg-brand-dark dark:bg-brand-light text-brand-light dark:text-brand-dark font-display font-bold hover:-translate-y-1 hover:shadow-solid-dark dark:hover:shadow-solid-light transition-all border border-transparent dark:border-zinc-700">Subscribe</a>
     </div>
 
@@ -37,6 +41,9 @@
     {% for item in header_items %}
       <a href="{{ item.url }}" class="font-medium py-2 hover:text-brand-cyan transition-colors"{% if item.open_in_new_tab %} target="_blank" rel="noopener noreferrer"{% endif %}>{{ item.title }}</a>
     {% endfor %}
+    {% for category in category_nav_items %}
+      <a href="{{ category.url }}" class="font-medium py-2 hover:text-brand-cyan transition-colors {% if category.url in request.path %}text-brand-cyan{% endif %}" {% if category.url in request.path %}aria-current="page"{% endif %}>{{ category.name }}</a>
+    {% endfor %}
     <form method="post" action="/newsletter/subscribe/" data-newsletter-form class="space-y-2 pt-2 border-t border-zinc-200 dark:border-zinc-800" id="mobile-newsletter">
       {% csrf_token %}
       <input type="hidden" name="source" value="nav-mobile" />