feat: add platform detail/edit admin UI and service enhancements

- Add platform detail and edit admin pages with templates and JS
- Add ContentPageService methods: list_all_platform_pages, list_all_vendor_defaults
- Deprecate /admin/platform-homepage route (redirects to /admin/platforms)
- Add migration to fix content_page nullable columns
- Refine platform and vendor context middleware
- Add platform context middleware unit tests
- Update platforms.js with improved functionality
- Add section-based homepage plan documentation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/admin/partials/sidebar.html

@@ -105,7 +105,6 @@
     {{ section_header('Content Management', 'contentMgmt') }}
     {% call section_content('contentMgmt') %}
         {{ menu_item('platforms', '/admin/platforms', 'globe-alt', 'Platforms') }}
-        {{ menu_item('platform-homepage', '/admin/platform-homepage', 'home', 'Platform Homepage') }}
         {{ menu_item('content-pages', '/admin/content-pages', 'document-text', 'Content Pages') }}
         {{ menu_item('vendor-theme', '/admin/vendor-themes', 'color-swatch', 'Vendor Themes') }}
     {% endcall %}