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

lib/utils/downloads/download_manager.dart

@@ -4,6 +4,7 @@ import 'dart:io';
 
 import 'package:background_downloader/background_downloader.dart' as bd;
 import 'package:flutter/foundation.dart';
+import 'package:path_provider/path_provider.dart';
 
 import '../../api/marianumcloud/webdav/webdav_api.dart';
 import '../../notification/notification_service.dart';
@@ -34,6 +35,10 @@ class DownloadManager {
   /// "Teilen"/"Speichern" actions.
   static const _directory = 'downloads';
 
+  /// Where finished downloads land (`BaseDirectory.temporary` + [_directory]).
+  static Future<Directory> downloadsDirectory() async =>
+      Directory('${(await getTemporaryDirectory()).path}/$_directory');
+
   final Map<String, DownloadJob> _jobs = {}; // keyed by remotePath
   final Map<String, DownloadJob> _byTaskId = {};