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-lock.json

@@ -20,6 +20,7 @@
         "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": {
@@ -6469,6 +6470,12 @@
         "node": ">=20.0.0"
       }
     },
+    "node_modules/sortablejs": {
+      "version": "1.15.7",
+      "resolved": "https://registry.npmjs.org/sortablejs/-/sortablejs-1.15.7.tgz",
+      "integrity": "sha512-Kk8wLQPlS+yi1ZEf48a4+fzHa4yxjC30M/Sr2AnQu+f/MPwvvX9XjZ6OWejiz8crBsLwSq8GHqaxaET7u6ux0A==",
+      "license": "MIT"
+    },
     "node_modules/source-map": {
       "version": "0.5.7",
       "resolved": "https://registry.npmjs.org/source-map/-/source-map-0.5.7.tgz",