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/ai_assistant/plugin.py

@@ -41,7 +41,7 @@ class AIAssistantPlugin(BasePlugin):
         ],
         is_core=True,
         menu_items=[
-            FrontendMenuItem(label_key='nav.aiAssistant', label='AI Assistant', path='/ai-assistant', icon='Bot', order=90),
+            FrontendMenuItem(label_key='nav.aiAssistant', label='KI Assistent', path='/ai-assistant', icon='Bot', order=90),
         ],
         page_routes=[
             FrontendPageRoute(path='/ai-assistant', component='@/pages/AIAssistant', protected=True),