added support for multiple accounts, guardian login bugfixes, ui changes

lib/widget/user_avatar.dart

@@ -162,7 +162,9 @@ Future<AvatarPayload?> _fetchAvatarPayload(String url) async {
     () => http.get(
       Uri.parse(url),
       headers: {
-        ...SessionManager().requireNextcloud().authHeaders,
+        // User avatars are public in Nextcloud, so the account switcher can
+        // show them while a guardian (no Nextcloud identity) is active.
+        ...?SessionManager().current?.nextcloud?.authHeaders,
         'Accept': 'image/png,image/jpeg,image/webp,image/svg+xml',
       },
     ),