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/pubspec.lock

@@ -41,6 +41,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.19.1"
+  crypto:
+    dependency: transitive
+    description:
+      name: crypto
+      sha256: c8ea0233063ba03258fbcf2ca4d6dadfefe14f02fab57702265467a19f27fadf
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.0.7"
   fake_async:
     dependency: transitive
     description:
@@ -349,6 +357,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.1.1"
+  web_socket:
+    dependency: transitive
+    description:
+      name: web_socket
+      sha256: "34d64019aa8e36bf9842ac014bb5d2f5586ca73df5e4d9bf5c936975cae6982c"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.0.1"
+  web_socket_channel:
+    dependency: "direct main"
+    description:
+      name: web_socket_channel
+      sha256: d645757fb0f4773d602444000a8131ff5d48c9e47adfe9772652dd1a4f2d45c8
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.0.3"
   xdg_directories:
     dependency: transitive
     description: