added verbose error body in case of errors

lib/api/marianumcloud/talk/talk_api.dart

@@ -60,7 +60,15 @@ abstract class TalkApi<T extends ApiResponse?> extends ApiRequest {
 
     final status = data.statusCode;
     if (status < 200 || status >= 300) {
-      final detail = 'Talk $endpoint -> HTTP $status';
+      // Talk's OCS errors put the real reason in the response body (e.g.
+      // expired session, removed participant, malformed reply target).
+      // Include a trimmed preview so the in-app error dialog and logs
+      // surface the cause instead of just the bare status code.
+      final body = data.body.replaceAll(RegExp(r'\s+'), ' ').trim();
+      final preview = body.length > 500 ? '${body.substring(0, 500)}…' : body;
+      final detail = body.isEmpty
+          ? 'Talk $endpoint -> HTTP $status'
+          : 'Talk $endpoint -> HTTP $status body=$preview';
       log(detail);
       if (status == 401) {
         throw AuthException.unauthorized(technicalDetails: detail);