feat(admin): separate platform CRUD from CMS, add entity selector macro

- Move platforms menu from CMS to Platform Admin section with create/edit
- Add platform create page, API endpoint, and service method
- Remove CMS-specific content from platform list and detail pages
- Create shared entity_selector + entity_selected_badge Jinja macros
- Create entity-selector.js generalizing store-selector.js for any entity
- Add Tom Select merchant filter to stores page with localStorage persistence
- Migrate store-products page to use shared macros (remove 53 lines of duped CSS)
- Fix broken icons: puzzle→puzzle-piece, building-storefront→store, language→translate, server→cube

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/marketplace/templates/marketplace/admin/letzshop-store-directory.html

@@ -44,7 +44,7 @@
                 <p class="text-2xl font-bold text-gray-900 dark:text-white" x-text="stats.total_stores || 0"></p>
             </div>
             <div class="w-10 h-10 bg-blue-100 dark:bg-blue-900/30 rounded-lg flex items-center justify-center">
-                <span x-html="$icon('building-storefront', 'w-5 h-5 text-blue-600 dark:text-blue-400')"></span>
+                <span x-html="$icon('store', 'w-5 h-5 text-blue-600 dark:text-blue-400')"></span>
             </div>
         </div>
     </div>
@@ -147,7 +147,7 @@
 <div x-show="!loading" x-cloak class="bg-white dark:bg-gray-800 rounded-lg border border-gray-200 dark:border-gray-700 overflow-hidden">
     <!-- Empty State -->
     <div x-show="stores.length === 0" class="text-center py-12">
-        <span x-html="$icon('building-storefront', 'w-12 h-12 mx-auto text-gray-400')"></span>
+        <span x-html="$icon('store', 'w-12 h-12 mx-auto text-gray-400')"></span>
         <h3 class="mt-4 text-lg font-medium text-gray-900 dark:text-white">No stores found</h3>
         <p class="mt-2 text-gray-500 dark:text-gray-400">
             <span x-show="stats.total_stores === 0">Click "Sync from Letzshop" to import stores.</span>