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/routes/pages/admin.py

@@ -491,6 +491,24 @@ async def admin_platforms_list(
     )
 
 
+@router.get(
+    "/platforms/create", response_class=HTMLResponse, include_in_schema=False
+)
+async def admin_platform_create(
+    request: Request,
+    current_user: User = Depends(require_menu_access("platforms", FrontendType.ADMIN)),
+    db: Session = Depends(get_db),
+):
+    """
+    Render platform creation form.
+    Allows creating a new platform with basic settings.
+    """
+    return templates.TemplateResponse(
+        "tenancy/admin/platform-create.html",
+        get_admin_context(request, db, current_user),
+    )
+
+
 @router.get(
     "/platforms/{platform_code}", response_class=HTMLResponse, include_in_schema=False
 )