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/tenancy/services/platform_service.py

@@ -500,6 +500,29 @@ class PlatformService:
 
         return None
 
+    @staticmethod
+    def create_platform(db: Session, data: dict) -> Platform:
+        """
+        Create a new platform.
+
+        Note: This method does NOT commit the transaction.
+        The caller (API endpoint) is responsible for committing.
+
+        Args:
+            db: Database session
+            data: Dictionary of fields for the new platform
+
+        Returns:
+            Created Platform object (with pending changes)
+        """
+        platform = Platform()
+        for field, value in data.items():
+            if hasattr(platform, field):
+                setattr(platform, field, value)
+        db.add(platform)
+        logger.info(f"[PLATFORMS] Created platform: {platform.code}")
+        return platform
+
     @staticmethod
     def update_platform(
         db: Session, platform: Platform, update_data: dict