Stabilize frontend login. Make chat messages multiline.

frontend/lib/backend.dart

@@ -24,7 +24,7 @@ class Backend {
   
   // Authentication
   static ({String username, String password})? _credentials;
-  static Completer? _loginCompleter;
+  static Completer<bool>? _loginCompleter;
   static Completer? _credentialsCompleter;
 
   static void setup(String url) {
@@ -49,8 +49,11 @@ class Backend {
     }
   }
 
-  static Future<void> auth(String username, String password) {
-    _socket!.send(json.encode({'username': username, 'password': password}));
+  static Future<bool> auth(String username, String password) {
+    if (_credentials != null) {
+      print("Cannot authenticate twice");
+      return Future.value(false);
+    }
     _credentials = (username: username, password: password);
     if (_credentialsCompleter != null) {
       _credentialsCompleter!.complete(_credentials);
@@ -62,11 +65,13 @@ class Backend {
 
   static void connect() {
     _socket = WebSocket(_url! + "/ws");
-    _socket!.onOpen.listen((event) {
+    _socket!.onOpen.listen((event) async {
       print('Connected to the WebSocket server');
       Backend.invalidateCache(chat: true);
 
       print('Logging in...');
+      // secure the listener for the response before sending credentials
+      var responseFuture = nextIncomingMessage();
       if (_credentials != null) {
         _socket!.send(json.encode({'username': _credentials!.username, 'password': _credentials!.password}));
       } else {
@@ -75,7 +80,14 @@ class Backend {
           _socket!.send(json.encode({'username': credentials.username, 'password': credentials.password}));
         });
       }
-      _loginCompleter?.complete();
+      var response = await responseFuture;
+      if (response['error'] == null) {
+        _loginCompleter?.complete(true);
+      } else {
+        _loginCompleter?.complete(false);
+        _socket!.close(); // Retry
+      }
+      _loginCompleter = null;
     });
     _socket!.onError.listen((event) {
       print('Error: $event');
@@ -92,11 +104,10 @@ class Backend {
 
   static void disconnect() {
     _socket?.close();
-    _loginCompleter = null;
   }
 
   static void _reconnect() {
-    Future.delayed(Duration(seconds: 5), () {
+    Future.delayed(Duration(seconds: 1), () {
       connect();
     });
   }
@@ -140,10 +151,17 @@ class Backend {
         print("No active listeners for chatId $chatId");
       }
     } else {
+      if (jsonData['error'] != null) {
+        print("Error: ${jsonData['error']}");
+      }
       _packetStreamController.add(jsonData);
     }
   }
 
+  static Future<Map<String, dynamic>> nextIncomingMessage() {
+    return _packetStreamController.stream.first;
+  }
+
   static Stream<Map<String, dynamic>> chatMessages(int chatId) {
     if (!_chatControllers.containsKey(chatId) || _chatControllers[chatId]!.isClosed) {
       _chatControllers[chatId] = StreamController<Map<String, dynamic>>.broadcast();