feat: Wahl result view + live WebSocket chat in the Flutter client

Backend:
- fix(chat): ChatGateway stored the per-socket caller only after the async
  token check resolved, so a client that sent chat:join immediately on open
  raced ahead and got 4001. The caller is now stored as a promise that the
  message handlers await. Verified with a two-client send/receive E2E test
  against local Postgres.

Client (client/app/):
- Wahl screen gains a "Ergebnis" tab backed by GET /wahl/guest/results
  (PENDING / ASSIGNED with workshop + wish rank / UNASSIGNED).
- Chat channel view loads history over REST, then connects the /chat
  WebSocket (chat_socket.dart): live chat:message stream + a compose bar
  that sends chat:send. Shows the socket status.
- web_socket_channel dependency added.

flutter analyze/test/build web all green; backend npm test 56.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

client/app/lib/chat_socket.dart

@@ -0,0 +1,57 @@
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:web_socket_channel/web_socket_channel.dart';
+
+import 'api.dart';
+
+/// Thin wrapper over the raw `ws` chat gateway. The NestJS `WsAdapter`
+/// expects `{"event": ..., "data": ...}` frames in both directions.
+class ChatSocket {
+  ChatSocket(this._uri);
+  final Uri _uri;
+
+  WebSocketChannel? _channel;
+  final _messages = StreamController<ChatMessage>.broadcast();
+  final _status = StreamController<String>.broadcast();
+
+  /// Incoming `chat:message` frames.
+  Stream<ChatMessage> get messages => _messages.stream;
+
+  /// "connected" / "closed" / "error: ..." for a small status line.
+  Stream<String> get status => _status.stream;
+
+  void connect(String channelId) {
+    _channel = WebSocketChannel.connect(_uri);
+    _channel!.stream.listen(
+      (raw) {
+        _status.add('connected');
+        try {
+          final frame = jsonDecode(raw as String) as Map<String, dynamic>;
+          if (frame['event'] == 'chat:message') {
+            _messages.add(ChatMessage.fromJson(frame['data'] as Map<String, dynamic>));
+          }
+        } catch (_) {
+          // ignore frames we don't model
+        }
+      },
+      onError: (Object e) => _status.add('error: $e'),
+      onDone: () => _status.add('closed'),
+    );
+    _send('chat:join', {'channelId': channelId});
+  }
+
+  void sendMessage(String channelId, String body) {
+    _send('chat:send', {'channelId': channelId, 'body': body});
+  }
+
+  void _send(String event, Map<String, dynamic> data) {
+    _channel?.sink.add(jsonEncode({'event': event, 'data': data}));
+  }
+
+  void dispose() {
+    _channel?.sink.close();
+    _messages.close();
+    _status.close();
+  }
+}