feat: navigation restructure + drag-and-drop menu ordering

Navigation:
- Remove Plugins header from sidebar
- Merge static items and plugin items into unified sorted list
- Group related menu items (Dateien, E-Mail, Kalender, Automation)
- German labels for all menu items
- Sort by order field, alphabetical fallback for ties

Drag-and-Drop Menu:
- New backend endpoints: GET/PUT /api/v1/users/me/menu-order
- Store menu order in user preferences JSONB field
- New SettingsMenuOrder page with @dnd-kit drag-and-drop
- New Settings tab: Menu ordering
- Sidebar reads saved menu order and sorts accordingly
- Reset to default option

frontend/src/api/users.ts

@@ -3,7 +3,7 @@
  */
 
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
-import { apiGet, apiPost, apiPatch, apiDelete } from './client';
+import { apiGet, apiPost, apiPut, apiPatch, apiDelete } from './client';
 import { PaginatedResponse } from './types';
 
 // ── Types matching backend schemas ──
@@ -80,6 +80,29 @@ export function useUpdateUser() {
   });
 }
 
+// ── Menu Order ──
+
+export interface MenuOrderResponse {
+  menu_order: string[];
+}
+
+export function useMenuOrder() {
+  return useQuery({
+    queryKey: ['menu-order'],
+    queryFn: () => apiGet<MenuOrderResponse>('/users/me/menu-order'),
+  });
+}
+
+export function useUpdateMenuOrder() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: (menu_order: string[]) => apiPut('/users/me/menu-order', { menu_order }),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['menu-order'] });
+    },
+  });
+}
+
 export function useDeleteUser() {
   const queryClient = useQueryClient();
   return useMutation({