feat: standalone buttons for all plugin pages + window system for modals

Standalone buttons:
- Add ExternalLink button to Dms, Calendar, Mail, ContactsList toolbars
- Create standalone pages for each (no sidebar, full screen)
- Add standalone routes outside ProtectedRoute

Window system for modals:
- AppointmentModal -> AppointmentEditForm + openWindow()
- ComposeModal -> MailComposeForm + openWindow()
- FilePreviewModal -> FilePreviewContent + openWindow()
- Calendar, Mail, Dms use openWindow() instead of modal state

frontend/src/routes/index.tsx

@@ -37,6 +37,10 @@ const ProactiveAISettings = React.lazy(() => import('@/pages/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 DmsStandalonePage = React.lazy(() => import('@/pages/DmsStandalone').then(m => ({ default: m.DmsStandalonePage })));
+const CalendarStandalonePage = React.lazy(() => import('@/pages/CalendarStandalone').then(m => ({ default: m.CalendarStandalonePage })));
+const MailStandalonePage = React.lazy(() => import('@/pages/MailStandalone').then(m => ({ default: m.MailStandalonePage })));
+const ContactsStandalonePage = React.lazy(() => import('@/pages/ContactsStandalone').then(m => ({ default: m.ContactsStandalonePage })));
 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 })));
@@ -70,6 +74,22 @@ const router = createBrowserRouter([
     path: '/ai-assistant-standalone',
     element: withSuspense(<AIAssistantStandalonePage />),
   },
+  {
+    path: '/dms-standalone',
+    element: withSuspense(<DmsStandalonePage />),
+  },
+  {
+    path: '/calendar-standalone',
+    element: withSuspense(<CalendarStandalonePage />),
+  },
+  {
+    path: '/mail-standalone',
+    element: withSuspense(<MailStandalonePage />),
+  },
+  {
+    path: '/contacts-standalone',
+    element: withSuspense(<ContactsStandalonePage />),
+  },
   {
     path: '/password-reset',
     element: <PasswordResetRequestPage />,