fix: loyalty module end-to-end — merchant route, store menus, sidebar, API error handling

- Add merchant loyalty overview route and template (was 404)
- Fix store loyalty route paths to match menu URLs (/{store_code}/loyalty/...)
- Add loyalty rewards card to storefront account dashboard
- Fix merchant overview to resolve merchant via get_merchant_for_current_user_page
- Fix store login to use store's primary platform for JWT token (interim fix)
- Fix apiClient to attach status/errorCode to thrown errors (fixes error.status
  checks in 12+ JS files — loyalty settings, terminal, email templates, etc.)
- Hide "Add Product" sidebar button when catalog module is not enabled
- Add proposal doc for proper platform detection in store login flow

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

static/shared/js/api-client.js

@@ -150,7 +150,10 @@ class APIClient {
 
                 const errorMessage = data.message || data.detail || 'Unauthorized - please login again';
                 apiLog.error('Throwing authentication error:', errorMessage);
-                throw new Error(errorMessage);
+                const authError = new Error(errorMessage);
+                authError.status = response.status;
+                authError.errorCode = data.error_code;
+                throw authError;
             }
 
             // Handle non-OK responses
@@ -161,7 +164,10 @@ class APIClient {
                     message: errorMessage,
                     errorCode: data.error_code
                 });
-                throw new Error(errorMessage);
+                const apiError = new Error(errorMessage);
+                apiError.status = response.status;
+                apiError.errorCode = data.error_code;
+                throw apiError;
             }
 
             apiLog.info('Request completed successfully');