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/dms/plugin.py

@@ -32,8 +32,8 @@ class DmsPlugin(BasePlugin):
             "dms:admin",
         ],
         menu_items=[
-            FrontendMenuItem(label_key='nav.files', label='Files', path='/dms', icon='FolderOpen', order=40),
-            FrontendMenuItem(label_key='nav.trash', label='Trash', path='/dms/trash', icon='Trash2', group='nav.files', order=41),
+            FrontendMenuItem(label_key='nav.dms', label='Dateien', path='/dms', icon='FolderOpen', group='Dateien', order=40),
+            FrontendMenuItem(label_key='nav.dms.trash', label='Papierkorb', path='/dms/trash', icon='Trash2', group='Dateien', order=41),
         ],
         page_routes=[
             FrontendPageRoute(path='/dms', component='@/pages/Dms', protected=True),