Portals shut down without crashing on the way out

When a portal restarted, it could release the memory holding your workspace's compiled views while background work was still using it. Everything had already finished successfully by then, so the only sign was a crash right at the end of shutdown — untidy, and it made restarts harder to tell apart from real faults.

Shutdown now stops that background work and waits for it to finish before letting go of anything, so a restart ends cleanly.