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>

backend/src/chat/chat.gateway.ts

@@ -15,10 +15,14 @@ import { ChatCaller, ChatService } from './chat.service';
 /// Raw `ws` gateway (no socket.io rooms available), so channel membership is
 /// tracked manually per connected socket. Auth happens once at handshake via
 /// a `?token=` query param since passport guards don't run for WS upgrades.
+///
+/// The per-socket caller is stored as a *promise*: the token check is async
+/// and a client can send `chat:join` before it resolves, so handlers await
+/// the stored promise instead of assuming it's already populated.
 @WebSocketGateway({ path: '/chat' })
 export class ChatGateway implements OnGatewayConnection, OnGatewayDisconnect {
   private readonly logger = new Logger(ChatGateway.name);
-  private readonly callers = new WeakMap<WebSocket, ChatCaller>();
+  private readonly callers = new Map<WebSocket, Promise<ChatCaller>>();
   private readonly rooms = new Map<string, Set<WebSocket>>();
 
   constructor(
@@ -26,18 +30,18 @@ export class ChatGateway implements OnGatewayConnection, OnGatewayDisconnect {
     private readonly chat: ChatService,
   ) {}
 
-  async handleConnection(client: WebSocket, request: IncomingMessage) {
+  handleConnection(client: WebSocket, request: IncomingMessage) {
     const token = new URL(request.url ?? '', 'http://localhost').searchParams.get('token');
     if (!token) {
       client.close(4001, 'Missing token');
       return;
     }
-    try {
-      this.callers.set(client, await this.tokenVerification.verifyEither(token));
-    } catch (err) {
+    const pending = this.tokenVerification.verifyEither(token).catch((err) => {
       this.logger.warn(`WS auth failed: ${(err as Error).message}`);
       client.close(4001, 'Unauthorized');
-    }
+      throw err;
+    });
+    this.callers.set(client, pending);
   }
 
   handleDisconnect(client: WebSocket) {
@@ -52,7 +56,7 @@ export class ChatGateway implements OnGatewayConnection, OnGatewayDisconnect {
     @ConnectedSocket() client: WebSocket,
     @MessageBody() data: { channelId: string },
   ) {
-    const caller = this.requireCaller(client);
+    const caller = await this.resolveCaller(client);
     await this.chat.assertCanRead(data.channelId, caller);
     this.roomFor(data.channelId).add(client);
     return { event: 'chat:joined', data: { channelId: data.channelId } };
@@ -63,19 +67,24 @@ export class ChatGateway implements OnGatewayConnection, OnGatewayDisconnect {
     @ConnectedSocket() client: WebSocket,
     @MessageBody() data: { channelId: string; body: string },
   ) {
-    const caller = this.requireCaller(client);
+    const caller = await this.resolveCaller(client);
     const message = await this.chat.sendMessage(data.channelId, caller, data.body);
     this.broadcast(data.channelId, { event: 'chat:message', data: message });
     return { event: 'chat:sent', data: { id: message.id } };
   }
 
-  private requireCaller(client: WebSocket): ChatCaller {
-    const caller = this.callers.get(client);
-    if (!caller) {
+  private async resolveCaller(client: WebSocket): Promise<ChatCaller> {
+    const pending = this.callers.get(client);
+    if (!pending) {
+      client.close(4001, 'Unauthorized');
+      throw new Error('Unauthorized WS client');
+    }
+    try {
+      return await pending;
+    } catch {
       client.close(4001, 'Unauthorized');
       throw new Error('Unauthorized WS client');
     }
-    return caller;
   }
 
   private roomFor(channelId: string): Set<WebSocket> {