Add pregame lobby with live chat

Players used to get bounced back to the dashboard when a session
wasn't full yet. Now they land on a lobby page showing who has
joined, and can chat with each other while waiting - messages are
broadcast live over the existing Mercure hub, and the session
auto-starts (and the lobby notifies everyone) once it fills up.

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

src/Game/Controller/GameController.php

@@ -141,12 +141,21 @@ final class GameController extends AbstractController
             return $this->redirectToRoute('game', ['session' => $session->getId()]);
         }
 
+        if ($request->isMethod('POST') && $request->request->has('send_message')) {
+            $dashboardService->postLobbyMessage($session, $user, (string) $request->request->get('content', ''));
+            return $this->redirectToRoute('game', ['session' => $session->getId()]);
+        }
+
         // Lazily pick up readiness changes from other players since our last request
         $dashboardService->checkAllPlayersReady($session);
 
         if ($session->getStatus() === SessionStatus::CREATED) {
-            $this->addFlash('info', 'This session is still waiting for more players to join.');
-            return $this->redirectToRoute('game_dashboard');
+            return $this->render('game/lobby.html.twig', [
+                'session' => $session,
+                'messages' => $dashboardService->getLobbyMessages($session),
+                'player' => $player,
+                'mercure_public_url' => $this->mercurePublicUrl,
+            ]);
         }
 
         if ($session->getStatus() === SessionStatus::WON) {