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/templates/admin/base.html

@@ -141,6 +141,9 @@
     <!-- 7. SEVENTH: Store Selector (depends on Tom Select and API Client) -->
     <script defer src="{{ url_for('core_static', path='shared/js/store-selector.js') }}"></script>
 
+    <!-- 7b. Entity Selector (generic version of store selector) -->
+    <script defer src="{{ url_for('core_static', path='shared/js/entity-selector.js') }}"></script>
+
     <!-- 8a. Alpine.js Collapse Plugin (must load before Alpine) -->
     <script defer src="https://cdn.jsdelivr.net/npm/@alpinejs/collapse@3.13.3/dist/cdn.min.js"></script>