backend: separated agent classes. frontend: chat message context menu (does nothing)

frontend/lib/backend.dart

@@ -49,7 +49,7 @@ class Backend {
     }
   }
 
-  static Future<bool> auth(String username, String password) {
+  static Future<bool> auth(String username, String password) async {
     if (_credentials != null) {
       print("Cannot authenticate twice");
       return Future.value(false);
@@ -60,7 +60,11 @@ class Backend {
       _credentialsCompleter = null;
     }
     _loginCompleter = Completer();
-    return _loginCompleter!.future;
+    final success = await _loginCompleter!.future;
+    if (!success) {
+      _credentials = null;
+    }
+    return success;
   }
 
   static void connect() {