gpui: Move 'app closing on last window closed' behavior to app-side (#41436)

This commit is a continuation of #36548. As per [mikayla-maki's
Comment](https://github.com/zed-industries/zed/pull/36548#issuecomment-3412140698),
I removed the process management behavior located in GPUI and
reimplemented it in Zed.

Release Notes:

- N/A

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Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>
ddf5937899Tryanks committed on 11/10/2025, 5:19:35 PM· committed by GitHubparent6e1d86f
4 files changedLine totals unavailable