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/automation/plugin.py

@@ -75,13 +75,15 @@ class AutomationPlugin(BasePlugin):
                 label="Automation",
                 path="/automation",
                 icon="Zap",
+                group="Automation",
                 order=50,
             ),
             FrontendMenuItem(
                 label_key="nav.agents",
-                label="Agents",
+                label="Agenten",
                 path="/agents",
                 icon="Bot",
+                group="Automation",
                 order=51,
             ),
         ],