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/pages/ContactsList.tsx

@@ -18,7 +18,7 @@ import { ContactDetail } from '@/components/contacts/ContactDetail';
 import { ContactEditForm } from '@/components/contacts/ContactEditForm';
 import { useWindowStore } from '@/store/windowStore';
 import { SavedFilters } from '@/components/SavedFilters';
-import { ArrowDownAZ, ArrowUpZA, ChevronLeft, LayoutGrid, List, Plus } from 'lucide-react';
+import { ArrowDownAZ, ArrowUpZA, ChevronLeft, ExternalLink, LayoutGrid, List, Plus } from 'lucide-react';
 import {
   useUnifiedContacts,
   useUnifiedContact,
@@ -201,6 +201,17 @@ export function ContactsListPage() {
         ),
         onClick: handleCreate,
       },
+      {
+        id: 'open-standalone',
+        plugin: 'contacts',
+        label: 'In neuem Fenster',
+        type: 'button' as const,
+        group: 'actions',
+        icon: (
+          <ExternalLink className="w-3.5 h-3.5" strokeWidth={2} />
+        ),
+        onClick: () => window.open('/contacts-standalone', '_blank', 'width=1200,height=800'),
+      },
     ];
     registerItems('contacts', items);
     return () => unregisterPlugin('contacts');