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 Unpublished pages are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/bluesky.html

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+<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="M2.25 15a4.5 4.5 0 0 0 4.5 4.5H18a3.75 3.75 0 0 0 1.332-7.257 3 3 0 0 0-3.758-3.848 5.25 5.25 0 0 0-10.233 2.33A4.502 4.502 0 0 0 2.25 15Z" /></svg>

templates/components/icons/github.html

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+<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="M17.25 6.75 22.5 12l-5.25 5.25m-10.5 0L1.5 12l5.25-5.25m7.5-3-4.5 16.5" /></svg>

templates/components/icons/linkedin.html

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+<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="M20.25 14.15v4.25c0 1.094-.787 2.036-1.872 2.18-2.087.277-4.216.42-6.378.42s-4.291-.143-6.378-.42c-1.085-.144-1.872-1.086-1.872-2.18v-4.25m16.5 0a2.18 2.18 0 0 0 .75-1.661V8.706c0-1.081-.768-2.015-1.837-2.175a48.114 48.114 0 0 0-3.413-.387m4.5 8.006c-.194.165-.42.295-.673.38A23.978 23.978 0 0 1 12 15.75c-2.648 0-5.195-.429-7.577-1.22a2.016 2.016 0 0 1-.673-.38m0 0A2.18 2.18 0 0 1 3 12.489V8.706c0-1.081.768-2.015 1.837-2.175a48.111 48.111 0 0 1 3.413-.387m7.5 0V5.25A2.25 2.25 0 0 0 13.5 3h-3a2.25 2.25 0 0 0-2.25 2.25v.894m7.5 0a48.667 48.667 0 0 0-7.5 0M12 12.75h.008v.008H12v-.008Z" /></svg>

templates/components/icons/mastodon.html

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+<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>

templates/components/icons/rss.html

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+<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="M12.75 19.5v-.75a7.5 7.5 0 0 0-7.5-7.5H4.5m0-6.75h.75c7.87 0 14.25 6.38 14.25 14.25v.75M6 18.75a.75.75 0 1 1-1.5 0 .75.75 0 0 1 1.5 0Z" /></svg>

templates/components/icons/twitter.html

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+<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="M6.633 10.25c.806 0 1.533-.446 2.031-1.08a9.041 9.041 0 0 1 2.861-2.4c.723-.384 1.35-.956 1.653-1.715a4.498 4.498 0 0 0 .322-1.672V2.75a.75.75 0 0 1 .75-.75 2.25 2.25 0 0 1 2.25 2.25c0 1.152-.26 2.243-.723 3.218-.266.558.107 1.282.725 1.282m0 0h3.126c1.026 0 1.945.694 2.054 1.715.045.422.068.85.068 1.285a11.95 11.95 0 0 1-2.649 7.521c-.388.482-.987.729-1.605.729H13.48c-.483 0-.964-.078-1.423-.23l-3.114-1.04a4.501 4.501 0 0 0-1.423-.23H5.904m10.598-9.75H14.25M5.904 18.5c.083.205.173.405.27.602.197.4-.078.898-.523.898h-.908c-.889 0-1.713-.518-1.972-1.368a12 12 0 0 1-.521-3.507c0-1.553.295-3.036.831-4.398C3.387 9.953 4.167 9.5 5 9.5h1.053c.472 0 .745.556.5.96a8.958 8.958 0 0 0-1.302 4.665c0 1.194.232 2.333.654 3.375Z" /></svg>

templates/components/icons/youtube.html

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+<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>