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/platforms.html

@@ -89,9 +89,17 @@
                     </a>
                     <div class="flex space-x-2">
                         <a
-                            :href="`/admin/content-pages?platform=${platform.code}`"
+                            :href="`/admin/content-pages?platform_code=${platform.code}&slug=home`"
                             class="inline-flex items-center px-3 py-1.5 text-xs font-medium text-gray-700 bg-white border border-gray-300 rounded-lg hover:bg-gray-50 dark:bg-gray-700 dark:text-gray-300 dark:border-gray-600 dark:hover:bg-gray-600"
-                            title="View content pages for this platform"
+                            title="Edit platform homepage"
+                        >
+                            <span x-html="$icon('home', 'w-4 h-4 mr-1')"></span>
+                            Homepage
+                        </a>
+                        <a
+                            :href="`/admin/content-pages?platform_code=${platform.code}`"
+                            class="inline-flex items-center px-3 py-1.5 text-xs font-medium text-gray-700 bg-white border border-gray-300 rounded-lg hover:bg-gray-50 dark:bg-gray-700 dark:text-gray-300 dark:border-gray-600 dark:hover:bg-gray-600"
+                            title="View all content pages for this platform"
                         >
                             <span x-html="$icon('document-text', 'w-4 h-4 mr-1')"></span>
                             Pages
@@ -101,8 +109,8 @@
                             class="inline-flex items-center px-3 py-1.5 text-xs font-medium text-white bg-purple-600 rounded-lg hover:bg-purple-700"
                             title="Edit platform settings"
                         >
-                            <span x-html="$icon('edit', 'w-4 h-4 mr-1')"></span>
-                            Edit
+                            <span x-html="$icon('cog', 'w-4 h-4 mr-1')"></span>
+                            Settings
                         </a>
                     </div>
                 </div>