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/calendar/plugin.py

@@ -42,8 +42,8 @@ class CalendarPlugin(BasePlugin):
             "calendar:admin",
         ],
         menu_items=[
-            FrontendMenuItem(label_key='nav.calendar', label='Calendar', path='/calendar', icon='Calendar', order=20),
-            FrontendMenuItem(label_key='nav.calendarKanban', label='Calendar Kanban', path='/calendar/kanban', icon='KanbanSquare', group='nav.calendar', order=21),
+            FrontendMenuItem(label_key='nav.calendar', label='Kalender', path='/calendar', icon='Calendar', group='Kalender', order=20),
+            FrontendMenuItem(label_key='nav.calendar.kanban', label='Kanban', path='/calendar/kanban', icon='KanbanSquare', group='Kalender', order=21),
         ],
         page_routes=[
             FrontendPageRoute(path='/calendar', component='@/pages/Calendar', protected=True),