fixed stale state after logout and replayed or lost share intents

lib/api/marianumcloud/talk/share_files_to_chat.dart

@@ -17,6 +17,10 @@ Future<void>? _shareFolderReady;
 Future<void> ensureTalkShareFolder() =>
     _shareFolderReady ??= _createTalkShareFolder();
 
+/// The folder is per user; after a sign-out the next account has to check
+/// again.
+void resetTalkShareFolderCache() => _shareFolderReady = null;
+
 Future<void> _createTalkShareFolder() async {
   try {
     final webdav = await WebdavApi.webdav;