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/kommunikation/plugin.py

@@ -50,7 +50,7 @@ class KommunikationPlugin(BasePlugin):
         ],
         is_core=True,
         menu_items=[
-            FrontendMenuItem(label_key='nav.communication', label='Communication', path='/communication', icon='MessageSquare', order=80),
+            FrontendMenuItem(label_key='nav.communication', label='Kommunikation', path='/communication', icon='MessageSquare', order=80),
         ],
         page_routes=[
             FrontendPageRoute(path='/communication', component='@/pages/Communication', protected=True),