feat: replace hardcoded navigation with CMS-managed models

Replace static nav/footer links with Wagtail-managed NavigationMenuItem
and SocialMediaLink orderables on SiteSettings. Unpublished pages are
automatically excluded from rendering, fixing the dead-link problem.

- Extend SiteSettings with site_name, tagline, footer_description,
  copyright_text branding fields
- Add NavigationMenuItem orderable (link_page/link_url, show_in_header,
  show_in_footer, sort_order) with automatic live-page filtering
- Add SocialMediaLink orderable with platform icon templates
- New template tags: get_nav_items, get_social_links
- Update nav.html and footer.html to render from CMS data
- Data migration seeds existing hardcoded values for zero-change deploy
- Update seed_e2e_content command for test/dev environments
- 18 new tests covering models, template tags, and rendered output

Closes #32

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

templates/components/nav.html

@@ -1,4 +1,5 @@
-{% load static %}
+{% load static core_tags %}
+{% get_nav_items "header" as header_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 -->
@@ -6,14 +7,14 @@
       <div class="w-8 h-8 bg-brand-dark dark:bg-brand-light text-brand-light dark:text-brand-dark flex items-center justify-center font-display font-bold text-xl group-hover:rotate-12 transition-transform">
         /
       </div>
-      <span class="font-display font-bold text-2xl tracking-tight">NO HYPE AI</span>
+      <span class="font-display font-bold text-2xl tracking-tight">{{ site_settings.site_name|default:"NO HYPE AI" }}</span>
     </a>
 
     <!-- Desktop Links -->
     <div class="hidden md:flex items-center gap-8 font-medium">
-      <a href="/" class="hover:text-brand-cyan transition-colors">Home</a>
-      <a href="/articles/" class="hover:text-brand-cyan transition-colors">Articles</a>
-      <a href="/about/" class="hover:text-brand-pink transition-colors">About</a>
+      {% 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 %}
       <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>
 
@@ -33,9 +34,9 @@
 <!-- Mobile Menu (outside <nav> to avoid duplicate form[data-newsletter-form] in nav scope) -->
 <div id="mobile-menu" class="md:hidden hidden sticky top-20 z-40 border-b border-zinc-200 dark:border-zinc-800 bg-brand-light/95 dark:bg-brand-dark/95 backdrop-blur-md">
   <div class="max-w-7xl mx-auto px-6 py-4 flex flex-col gap-4">
-    <a href="/" class="font-medium py-2 hover:text-brand-cyan transition-colors">Home</a>
-    <a href="/articles/" class="font-medium py-2 hover:text-brand-cyan transition-colors">Articles</a>
-    <a href="/about/" class="font-medium py-2 hover:text-brand-pink transition-colors">About</a>
+    {% 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 %}
     <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" />