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 Unpublished pages are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

@@ -140,11 +140,9 @@ class Command(BaseCommand):
         site.is_default_site = True
         site.save()
 
-        # Navigation menu items and social links — always reconcile to
+        # Navigation menu items and social links
-        # 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()
+        if not NavigationMenuItem.objects.filter(settings=settings).exists():
             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 = [
@@ -159,14 +157,11 @@ class Command(BaseCommand):
                 )
             if about_page:
                 nav_items.append(
-                NavigationMenuItem(
+                    NavigationMenuItem(settings=settings, link_page=about_page, link_title="About", sort_order=2)
-                    settings=settings, link_page=about_page,
-                    link_title="About", sort_order=2,
-                )
                 )
             NavigationMenuItem.objects.bulk_create(nav_items)
 
-        SocialMediaLink.objects.filter(settings=settings).delete()
+        if not SocialMediaLink.objects.filter(settings=settings).exists():
             SocialMediaLink.objects.bulk_create(
                 [
                     SocialMediaLink(