fix(servo-shell): pass app_id from LAUNCH_ACK to taskbar entry

system-ui.html: LAUNCH_ACK handler now passes msg.app_id to
ensureTaskbarEntry so the taskbar label shows the app name
immediately on launch, matching the behaviour of RUNNING_APPS.

infra/shell/system-ui.html

@@ -212,7 +212,7 @@
             ensureTaskbarEntry(s.session_id, s.app_id);
           });
         } else if (msg.type === 'LAUNCH_ACK') {
-          ensureTaskbarEntry(msg.session_id, null);
+          ensureTaskbarEntry(msg.session_id, msg.app_id || null);
         }
       }