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.

package.json

@@ -26,6 +26,7 @@
     "react-dom": "^19.2.8",
     "react-redux": "^9.3.0",
     "react-router-dom": "^7.18.2",
+    "sortablejs": "^1.15.7",
     "thumbhash": "^0.1.1"
   },
   "devDependencies": {