fix(config): add individual menu CRUD endpoints and fix frontend menu data handling

- Add POST /config/menus (create single menu)
- Add PUT /config/menus/{id} (update single menu)
- Add DELETE /config/menus/{id} (soft delete single menu)
- Frontend MenuConfig was calling individual CRUD routes that didn't exist,
  causing 404 errors on all create/update/delete operations
- Fix frontend to correctly handle nested tree response from backend

crates/erp-config/src/module.rs

@@ -56,7 +56,14 @@ impl ConfigModule {
             // Menu routes
             .route(
                 "/config/menus",
-                get(menu_handler::get_menus).put(menu_handler::batch_save_menus),
+                get(menu_handler::get_menus)
+                    .post(menu_handler::create_menu)
+                    .put(menu_handler::batch_save_menus),
+            )
+            .route(
+                "/config/menus/{id}",
+                put(menu_handler::update_menu)
+                    .delete(menu_handler::delete_menu),
             )
             // Setting routes
             .route(