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 Unpublished pages are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>

apps/core/management/commands/seed_e2e_content.py

@@ -7,6 +7,7 @@ from wagtail.models import Page, Site
 from apps.authors.models import Author
 from apps.blog.models import AboutPage, ArticleIndexPage, ArticlePage, HomePage, TagMetadata
 from apps.comments.models import Comment
+from apps.core.models import NavigationMenuItem, SiteSettings, SocialMediaLink
 from apps.legal.models import LegalIndexPage, LegalPage
 
 
@@ -139,4 +140,45 @@ class Command(BaseCommand):
         site.is_default_site = True
         site.save()
 
+        # Navigation menu items and social links — always reconcile to
+        # match the pages we just created (the data migration may have
+        # seeded partial items before these pages existed).
+        settings, _ = SiteSettings.objects.get_or_create(site=site)
+        NavigationMenuItem.objects.filter(settings=settings).delete()
+        article_index_page = ArticleIndexPage.objects.child_of(home).filter(slug="articles").first()
+        about_page = AboutPage.objects.child_of(home).filter(slug="about").first()
+        nav_items = [
+            NavigationMenuItem(settings=settings, link_page=home, link_title="Home", sort_order=0),
+        ]
+        if article_index_page:
+            nav_items.append(
+                NavigationMenuItem(
+                    settings=settings, link_page=article_index_page,
+                    link_title="Articles", sort_order=1,
+                )
+            )
+        if about_page:
+            nav_items.append(
+                NavigationMenuItem(
+                    settings=settings, link_page=about_page,
+                    link_title="About", sort_order=2,
+                )
+            )
+        NavigationMenuItem.objects.bulk_create(nav_items)
+
+        SocialMediaLink.objects.filter(settings=settings).delete()
+        SocialMediaLink.objects.bulk_create(
+            [
+                SocialMediaLink(
+                    settings=settings, platform="twitter",
+                    url="https://twitter.com/nohypeai",
+                    label="Twitter (X)", sort_order=0,
+                ),
+                SocialMediaLink(
+                    settings=settings, platform="rss",
+                    url="/feed/", label="RSS Feed", sort_order=1,
+                ),
+            ]
+        )
+
         self.stdout.write(self.style.SUCCESS("Seeded E2E content."))