Respect `scrollbar.show` setting in Markdown preview (#60410)

The markdown preview used `vertical_scrollbar_for()` which always sets
the scrollbar to auto-hide mode, ignoring the user's `scrollbar.show`
setting.

This fix uses
`Scrollbars::for_settings::<EditorSettingsScrollbarProxy>()` so the
markdown preview respects the same editor scrollbar visibility setting
as regular editor panes (always, auto, system, never).

Changes:

- Replaced `vertical_scrollbar_for()` with `custom_scrollbars()` +
`Scrollbars::for_settings::<EditorSettingsScrollbarProxy>()` in the
markdown preview render method

Fixes #60380

Release Notes:

- Fixed `scrollbar.show` setting not being respected in Markdown
previews

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Co-authored-by: Akshit Jain <akshitj11@users.noreply.github.com>
Co-authored-by: MrSubidubi <finn@zed.dev>