Defer world activation until welcome preflight confirmation

client/src/main.ts

@@ -2091,6 +2091,7 @@ async function onSignalingMessage(message: IncomingMessage): Promise<void> {
   }
   await onAppMessage(message);
   if (message.type === 'welcome') {
+    signaling.send({ type: 'welcome_ready' });
     await setupMediaAfterAuth();
     if (playSelfLoginSound) {
       void audio.playSample(SYSTEM_SOUND_URLS.logon, 1);

client/src/network/protocol.ts

@@ -367,6 +367,7 @@ export type OutgoingMessage =
   | { type: 'auth_login'; username: string; password: string }
   | { type: 'auth_resume'; sessionToken: string }
   | { type: 'auth_logout' }
+  | { type: 'welcome_ready' }
   | { type: 'admin_roles_list' }
   | { type: 'admin_role_create'; name: string }
   | { type: 'admin_role_update_permissions'; role: string; permissions: string[] }