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;

lib/api/marianumcloud/webdav/webdav_api.dart

@@ -18,7 +18,10 @@ abstract class WebdavApi<T> {
   /// changes (app password minted/renewed, account switch) so it never keeps
   /// authenticating with stale credentials.
   static Future<WebDavClient> get webdav {
-    final secret = AccountData().getNextcloudSecret();
+    // Keyed by user too: two accounts may share a password (no app password
+    // minted), and the client would keep the previous login name.
+    final secret =
+        '${AccountData().getUsername()}:${AccountData().getNextcloudSecret()}';
     if (_webdav == null || _webdavSecret != secret) {
       _webdavSecret = secret;
       _webdav = establishWebdavConnection();