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

frontend/src/pages/Settings.tsx

@@ -29,6 +29,7 @@ export function SettingsPage() {
     { to: '/settings/notifications', label: t('settings.notifications'), icon: '\ud83d\udd14' },
     { to: '/settings/theme', label: t('settings.theme', 'Theme'), icon: '\ud83c\udfa8' },
     { to: '/settings/mcp', label: t('settings.mcp', 'MCP'), icon: '\ud83d\udd27' },
+    { to: '/settings/menu', label: t('settings.menuOrder', 'Menü'), icon: '\ud83d\udccb' },
   ];
 
   const existingPaths = new Set(hardcodedNavItems.map(item => item.to));