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/routes/index.tsx

@@ -36,6 +36,7 @@ const AISettingsPage = React.lazy(() => import('@/pages/AISettings').then(m => (
 const ProactiveAISettings = React.lazy(() => import('@/pages/ProactiveAISettings').then(m => ({ default: m.ProactiveAISettings })));
 const SettingsThemePage = React.lazy(() => import('@/pages/SettingsTheme').then(m => ({ default: m.SettingsThemePage })));
 const SettingsMcpPage = React.lazy(() => import('@/pages/SettingsMcp').then(m => ({ default: m.SettingsMcpPage })));
+const SettingsMenuOrderPage = React.lazy(() => import('@/pages/SettingsMenuOrder').then(m => ({ default: m.SettingsMenuOrderPage })));
 const AutomationDashboardPage = React.lazy(() => import('@/pages/AutomationDashboard').then(m => ({ default: m.AutomationDashboardPage })));
 const AgentDashboardPage = React.lazy(() => import('@/pages/AgentDashboard').then(m => ({ default: m.AgentDashboardPage })));
 const AutomationSettingsPage = React.lazy(() => import('@/pages/AutomationSettings').then(m => ({ default: m.AutomationSettingsPage })));
@@ -113,6 +114,7 @@ const router = createBrowserRouter([
           { path: 'theme', element: withSuspense(<SettingsThemePage />) },
           { path: 'automation', element: withSuspense(<AutomationSettingsPage />) },
           { path: 'mcp', element: withSuspense(<SettingsMcpPage />) },
+          { path: 'menu', element: withSuspense(<SettingsMenuOrderPage />) },
           { path: '*', element: <PluginRouteRenderer /> },
         ],
       },