feat: add standalone AI assistant window button

- Add ExternalLink button to AI assistant toolbar
- New route /ai-assistant-standalone renders AIAssistantPage without sidebar
- Opens in new browser window via window.open

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 AIAssistantStandalonePage = React.lazy(() => import('@/pages/AIAssistantStandalone').then(m => ({ default: m.AIAssistantStandalonePage })));
 const SettingsAIPage = React.lazy(() => import('@/pages/SettingsAI').then(m => ({ default: m.SettingsAIPage })));
 const SettingsMenuOrderPage = React.lazy(() => import('@/pages/SettingsMenuOrder').then(m => ({ default: m.SettingsMenuOrderPage })));
 const SettingsStammdatenPage = React.lazy(() => import('@/pages/SettingsStammdaten').then(m => ({ default: m.SettingsStammdatenPage })));
@@ -65,6 +66,10 @@ const router = createBrowserRouter([
     path: '/login',
     element: <LoginPage />,
   },
+  {
+    path: '/ai-assistant-standalone',
+    element: withSuspense(<AIAssistantStandalonePage />),
+  },
   {
     path: '/password-reset',
     element: <PasswordResetRequestPage />,