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/store/categorySlice.js

@@ -168,6 +168,27 @@ export const renameCategory = createAsyncThunk(
   },
 );
 
+/**
+ * Reorder sibling categories (new order of children under the current parent).
+ * `PUT /api/category/order` → 200 { categoryIds: number[] }
+ */
+export const reorderCategories = createAsyncThunk(
+  'category/reorder',
+  async ({ categoryIds }, { rejectWithValue, getState }) => {
+    try {
+      const ids = requireArray(categoryIds, 'categoryIds');
+      const parentId = getState().category.currentCategoryId;
+      requireDefined(parentId, 'currentCategoryId');
+      const data = await category.reorder(parentId, ids);
+      requireObject(data, 'PUT /api/category/order');
+      requireArray(data.categoryIds, 'categoryIds');
+      return { parentId, categoryIds: requireArray(data.categoryIds, 'categoryIds') };
+    } catch (err) {
+      return catchBackendError(rejectWithValue)(err);
+    }
+  },
+);
+
 export const deleteCategory = createAsyncThunk(
   'category/delete',
   async (categoryId, { rejectWithValue }) => {
@@ -309,6 +330,26 @@ const categorySlice = createSlice({
             : row,
         );
       })
+      .addCase(reorderCategories.fulfilled, (state, action) => {
+        const { parentId, categoryIds } = action.payload;
+        if (!state.categories) return;
+        const parentKey = String(parentId);
+        const idToSort = new Map(
+          categoryIds.map((id, index) => [String(id), index]),
+        );
+        for (const node of Object.values(state.categories)) {
+          if (
+            node.parentId != null &&
+            String(node.parentId) === parentKey &&
+            idToSort.has(String(node.id))
+          ) {
+            state.categories[String(node.id)] = {
+              ...node,
+              sort: idToSort.get(String(node.id)),
+            };
+          }
+        }
+      })
       .addCase(createArticle.fulfilled, (state, action) => {
         const created = action.payload;
         if (created == null) return;