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/content-pages.html

@@ -32,7 +32,8 @@
             <div class="relative">
                 <select
                     x-model="selectedPlatform"
-                    class="pl-3 pr-8 py-2 text-sm text-gray-700 dark:text-gray-300 bg-gray-50 dark:bg-gray-700 border border-gray-300 dark:border-gray-600 rounded-lg focus:outline-none focus:border-purple-500 appearance-none cursor-pointer"
+                    class="pl-3 pr-8 py-2 text-sm text-gray-700 dark:text-gray-300 bg-gray-50 dark:bg-gray-700 border border-gray-300 dark:border-gray-600 rounded-lg focus:outline-none focus:border-purple-500 cursor-pointer"
+                    style="appearance: none; -webkit-appearance: none; -moz-appearance: none; background-image: none;"
                 >
                     <option value="">All Platforms</option>
                     <template x-for="platform in platforms" :key="platform.id">