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 Unpublished pages are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/youtube.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="m15.75 10.5 4.72-8.9a.75.75 0 0 0-.53-1.1H13.5l-1.5 3-1.5-3H4.06a.75.75 0 0 0-.53 1.1l4.72 8.9-4.72 8.9a.75.75 0 0 0 .53 1.1H10.5l1.5-3 1.5 3h6.44a.75.75 0 0 0 .53-1.1l-4.72-8.9Z" /></svg>