Add category rename and drag-and-drop reordering in edit mode

- Edit toggle behind the "Kategorien" title enables an edit mode.
- In edit mode all category tiles become inline rename fields (no outline,
  distinct background, small margins); articles and "new article" are hidden.
- Rename via PATCH /api/category/:id; reorder via PUT /api/category/order
  (parentId + categoryIds) with SortableJS.
- Only call backend when order actually changed; revert DOM order on error
  so further dragging keeps working.
- Redux: renameCategory + reorderCategories thunks, renameMode state.

src/api/index.js

@@ -345,6 +345,20 @@ export const category = {
       body: { name },
     });
   },
+
+  /**
+   * PUT /api/category/order
+   * Body: { parentId, categoryIds: number[] } — sibling order of the children
+   * under `parentId` (top-down). categoryIds must be exactly the current child
+   * set of parentId (same count, no foreign/duplicate ids) → 400 Invalid category order.
+   * → 200 { categoryIds: number[] }
+   */
+  reorder(parentId, categoryIds) {
+    return http('/api/category/order', {
+      method: 'PUT',
+      body: { parentId, categoryIds },
+    });
+  },
 };
 
 /**