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 Unpublished pages are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/icons/mastodon.html

@@ -0,0 +1 @@
+<svg class="w-4 h-4" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" stroke-width="1.5" stroke="currentColor"><path stroke-linecap="round" stroke-linejoin="round" d="M12 21a9.004 9.004 0 0 0 8.716-6.747M12 21a9.004 9.004 0 0 1-8.716-6.747M12 21c2.485 0 4.5-4.03 4.5-9S14.485 3 12 3m0 18c-2.485 0-4.5-4.03-4.5-9S9.515 3 12 3m0 0a8.997 8.997 0 0 1 7.843 4.582M12 3a8.997 8.997 0 0 0-7.843 4.582m15.686 0A11.953 11.953 0 0 1 12 10.5c-2.998 0-5.74-1.1-7.843-2.918m15.686 0A8.959 8.959 0 0 1 21 12c0 .778-.099 1.533-.284 2.253m0 0A17.919 17.919 0 0 1 12 16.5c-3.162 0-6.133-.815-8.716-2.247m0 0A9.015 9.015 0 0 1 3 12c0-1.605.42-3.113 1.157-4.418" /></svg>