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/index.css

@@ -10,3 +10,15 @@ body {
   overscroll-behavior-y: none;
   touch-action: manipulation;
 }
+
+/* SortableJS: Kategorie-Reorder im Edit-Modus */
+.sortable-ghost {
+  outline: 2px dashed #2e7d32;
+  outline-offset: 2px;
+  background-color: rgba(46, 125, 50, 0.12) !important;
+}
+
+.sortable-drag {
+  opacity: 1 !important;
+  box-shadow: 0 8px 20px rgba(0, 0, 0, 0.28);
+}