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/Dms.tsx

@@ -20,11 +20,12 @@ import { SourceTree } from '@/components/dms/SourceTree';
 import { FileExplorer, type ViewMode, type SortBy, type SortOrder } from '@/components/dms/FileExplorer';
 import { FileDetails } from '@/components/dms/FileDetails';
 import { UploadDropzone } from '@/components/dms/UploadDropzone';
-import { FilePreviewModal } from '@/components/dms/FilePreviewModal';
+import { FilePreviewContent } from '@/components/dms/FilePreviewContent';
+import { useWindowStore } from '@/store/windowStore';
 import { ShareDialog } from '@/components/dms/ShareDialog';
 import { BulkActions } from '@/components/dms/BulkActions';
 import { usePluginToolbarStore } from '@/store/pluginToolbarStore';
-import { ArrowRight, ChevronLeft, ChevronRight, Info, Plus, Trash2, Upload } from 'lucide-react';
+import { ArrowRight, ChevronLeft, ChevronRight, ExternalLink, Info, Plus, Trash2, Upload } from 'lucide-react';
 import {
   fetchFolders,
   createFolder,
@@ -72,7 +73,7 @@ export function DmsPage() {
   // Modal state
   const [showUpload, setShowUpload] = useState(false);
   const [showNewFolder, setShowNewFolder] = useState(false);
-  const [previewFile, setPreviewFile] = useState<DmsFile | null>(null);
+  const openWindow = useWindowStore((s) => s.openWindow);
   const [shareFile, setShareFile] = useState<DmsFile | null>(null);
   const [deleteTarget, setDeleteTarget] = useState<DmsFile | null>(null);
   const [submittingFolder, setSubmittingFolder] = useState(false);
@@ -195,8 +196,15 @@ export function DmsPage() {
 
   // Handle file double click (open preview)
   const handleFileDoubleClick = useCallback((file: DmsFile) => {
-    setPreviewFile(file);
-  }, []);
+    openWindow({
+      title: file.name,
+      type: 'file-preview',
+      component: FilePreviewContent,
+      componentProps: {
+        file,
+      },
+    });
+  }, [openWindow]);
 
   // Handle file share
   const handleFileShare = useCallback((file: DmsFile) => {
@@ -210,8 +218,15 @@ export function DmsPage() {
 
   // Handle file preview
   const handleFilePreview = useCallback((file: DmsFile) => {
-    setPreviewFile(file);
-  }, []);
+    openWindow({
+      title: file.name,
+      type: 'file-preview',
+      component: FilePreviewContent,
+      componentProps: {
+        file,
+      },
+    });
+  }, [openWindow]);
 
   // Handle toggle select
   const handleToggleSelect = useCallback((fileId: string) => {
@@ -439,6 +454,17 @@ export function DmsPage() {
         ),
         onClick: () => setShowDetails((v) => !v),
       },
+      {
+        id: 'open-standalone',
+        plugin: 'dms',
+        label: 'In neuem Fenster',
+        type: 'button' as const,
+        group: 'actions',
+        icon: (
+          <ExternalLink className="w-3.5 h-3.5" strokeWidth={2} />
+        ),
+        onClick: () => window.open('/dms-standalone', '_blank', 'width=1200,height=800'),
+      },
     ];
 
     // Add bulk actions when files are selected
@@ -706,13 +732,6 @@ export function DmsPage() {
         onCancel={() => setDeleteTarget(null)}
       />
 
-      {/* File preview modal */}
-      <FilePreviewModal
-        open={!!previewFile}
-        file={previewFile}
-        onClose={() => setPreviewFile(null)}
-      />
-
       {/* Share dialog */}
       <ShareDialog
         open={!!shareFile}