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/screens/home_screen.dart

@@ -34,7 +34,7 @@ class HomeScreen extends StatelessWidget {
         _NavTile(
           icon: Icons.forum,
           title: 'Chat',
-          subtitle: 'Kanäle & Nachrichten (lesen)',
+          subtitle: 'Kanäle, Verlauf & Live-Nachrichten',
           onTap: () => _open(context, ChatScreen(kcId: kcId)),
         ),
     ];