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

app/plugins/builtins/tasks/plugin.py

@@ -39,7 +39,7 @@ class TasksPlugin(BasePlugin):
         menu_items=[
             FrontendMenuItem(
                 label_key="nav.tasks",
-                label="Tasks",
+                label="Aufgaben",
                 path="/tasks",
                 icon="CheckSquare",
                 order=30,