Follow-up to https://github.com/zed-industries/zed/pull/60477
Closes #25022

We have more places that need a similar synchronization before quitting
the app: restarts on update, reloading the workspace.

On top of it, two more findings:

* `on_app_will_quit` was redundant: the global `on_app_quit` hook
flushes a strict superset of the same tasks and runs before windows are
cleared, while the entity hook held only a weak handle and could never
flush the `Window`-dependent workspace state anyway.

* previously, shutdown was run as `cx.spawn`, yet main threads got
parked on app shutdown: now using `cx.background_spawn` instead

Release Notes:

- Improved data persistence during various app shutdown cases

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Co-authored-by: Kostia R <k-bx@k-bx.com>