DMS redesign: 3-column explorer layout with toolbar, source tree, file details panel

- Add backend endpoints: GET /dms/files, GET /dms/folders/{id}/files
- New SourceTree component: sources (Meine Dateien, Geteilte Ordner, Externe Speicher)
- New FileExplorer component: 5 view modes (list, table, icons-sm/md/lg)
- New FileDetails component: right panel with file metadata and actions
- Rewrite Dms.tsx: PluginToolbar + 3-column ResizablePanel layout
- Update DmsFile type: size_bytes + uploaded_by (backward compat aliases)
- Add 60+ i18n keys for de/en

frontend/src/components/dms/FilePreviewModal.tsx

@@ -52,7 +52,7 @@ export function FilePreviewModal({ open, file, onClose }: FilePreviewModalProps)
       <div className="space-y-4">
         <div className="flex items-center gap-4 flex-wrap">
           <Badge variant="info">{file.mime_type.split('/')[1]?.toUpperCase() || t('dms.icon.other')}</Badge>
-          <span className="text-sm text-secondary-500">{formatFileSize(file.size)}</span>
+          <span className="text-sm text-secondary-500">{formatFileSize(file.size_bytes ?? file.size ?? 0)}</span>
           {file.created_at && (
             <span className="text-sm text-secondary-500">
               {t('dms.fileModified')}: {new Date(file.created_at).toLocaleDateString()}