Custom static pages CRUD (#28)

[nwwatson]

Summary

• Adds full CRUD for custom static pages with rich text (Lexxy) editor, autosave, and admin management
• Pages served at top-level URLs (/:slug) with catch-all route placed last to avoid conflicts
• Published pages with show_in_navigation enabled appear dynamically in the site header
• Migrates the hardcoded About page to a seed-data Page record; removes old about route

Changes

• Model: Page with Sluggable, Publishable, Navigable concerns; reserved slug validation
• Admin: Admin::PagesController with index/new/edit/create/update/destroy + JSON autosave
• Layout: admin_page_editor layout (simplified editor without AI/preview tabs)
• Public: PagesController#show with SEO meta tags; dynamic navigation in application layout
• Routes: Catch-all /:slug route moved to end of routes file; admin namespace includes resources :pages
• Sitemap: Updated to dynamically include all published pages
• Seeds: About and Contact pages created as seed data
• i18n: All page-related keys added to en.yml and es.yml
• Tests: Page model tests, public controller tests, admin controller tests, fixture updates
• Docs: CLAUDE.md and README.md updated

Testing

• All 631 tests pass
• Rubocop: no offenses on changed files
• Brakeman: no warnings
• Bundle audit / importmap audit: clean

Closes #28