story: Use the system menu bar on macOS (#2806)

Mac applications generally put their application menus in the global
menubar at the top of the screen.

This changes the story example to have MacOS draw the menubar at the top
of the screen, and also draw the title in the window, to behave more
like a typical mac application.

Closes #[issue number]

## Description

Describe in English for the changes made in this pull request and the
problem it solves.
Please keep **1 PR to solve 1 problem**, and keep **Small improvements
should be small modifications** to make PR easier to review and to
merge.

## Screenshot

### Before

https://l.e42.us/VVTYdCrh

### After

https://l.e42.us/TbVdmDWq

**Note**: This change does nothing on platforms other than macos right
now. I don't think a global menubar is currently common anywhere else,
although I'm sure some KDE configurations could prove me wrong. I've
tested Linux and Mac to ensure that the change is only active on Mac.
The Linux behavior was unchanged. I don't have a Windows machine to test
with, but I'm confident `cfg!(target_os = "macos")` will work as well
there as it did on Linux.

## Checklist

- [X] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [X] Reviewed the changes in this PR and confirmed AI generated code
(If any) is accurate.
- [X] Passed `cargo run` for story tests related to the changes.
- [X] Tested macOS, Windows and Linux platforms performance (if the
change is platform-specific)

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Co-authored-by: Floyd Wang <gassnake999@gmail.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>