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

lib/model/endpoint_data.dart

@@ -5,6 +5,9 @@ class Endpoint {
   Endpoint({required this.domain, this.path = ''});
 
   String full() => domain + path;
+
+  /// `https://` + [full], without a trailing slash.
+  String origin() => 'https://${full()}';
 }
 
 class EndpointData {

lib/model/session_wipe.dart

@@ -4,7 +4,6 @@ import 'dart:io';
 
 import 'package:firebase_messaging/firebase_messaging.dart';
 import 'package:flutter_app_badge/flutter_app_badge.dart';
-import 'package:path_provider/path_provider.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 import '../api/marianumcloud/talk/share_files_to_chat.dart';
@@ -53,7 +52,7 @@ abstract final class SessionWipe {
   /// unregister it).
   static Future<void> accountLeft() async {
     await _stepAsync('download files', () async {
-      final dir = Directory('${(await getTemporaryDirectory()).path}/downloads');
+      final dir = await DownloadManager.downloadsDirectory();
       if (dir.existsSync()) await dir.delete(recursive: true);
     });
     await _stepAsync(