consolidated downloads dir, nextcloud origin, upload route and chat timestamps; added webdav path tests

lib/widget/file_viewer/unknown_preview_block.dart

@@ -13,12 +13,12 @@ import '../app_progress_indicator.dart';
 /// trigger its `errorWidget`. Prefers `fileId` because the path variant
 /// is unreliable on some server configurations.
 String _ncPreviewUrl(RemoteFileRef remote, {int width = 1024}) {
-  final host = EndpointData().nextcloud().full();
+  final origin = EndpointData().nextcloud().origin();
   final id = remote.fileId;
   final selector = id != null
       ? 'fileId=$id'
       : 'file=${Uri.encodeQueryComponent(remote.path)}';
-  return 'https://$host/index.php/core/preview?$selector&x=$width&y=-1&a=1';
+  return '$origin/index.php/core/preview?$selector&x=$width&y=-1&a=1';
 }
 
 /// Header block for the "Vorschau nicht verfügbar" screen.