Global Styles: Group shadow editor rows as items

[ciampo]

What?

Closes #81861.

Fixes the shadow editor so each shadow row gets one separator.

Why?

ItemGroup styles expect the DOM shape produced by Item: a list-item wrapper around one inner surface. ShadowItem rendered a Dropdown directly, so the separator also targeted its remove button.

How?

Wrap each ShadowItem in a role="listitem" element, leaving the dropdown as the inner surface. This fixes the consumer without changing the generic ItemGroup selector.

Testing Instructions

1. Open Site Editor > Styles > Shadows.
2. Add and open a custom shadow, then add a second shadow row.
3. Hover a row and confirm there is one separator between rows and none across the remove button.

Testing Instructions for Keyboard

Repeat the steps with Tab and Enter. Confirm each remove button receives focus and removes only its row.

Screenshots

Use of AI Tools

Codex was used to analyze the issue, implement the change, and run verification.

[github-actions]

Size Change: -58 B (0%)

Total Size: 7.75 MB

📦 View Changed

Filename	Size	Change
build/scripts/editor/index.min.js	537 kB	+12 B (0%)
build/styles/edit-site/style-rtl.css	21.8 kB	-7 B (-0.03%)
build/styles/edit-site/style-rtl.min.css	17.9 kB	-8 B (-0.04%)
build/styles/edit-site/style.css	21.8 kB	-7 B (-0.03%)
build/styles/edit-site/style.min.css	17.9 kB	-8 B (-0.04%)
build/styles/editor/style-rtl.css	32 kB	-8 B (-0.03%)
build/styles/editor/style-rtl.min.css	27.3 kB	-12 B (-0.04%)
build/styles/editor/style.css	32 kB	-9 B (-0.03%)
build/styles/editor/style.min.css	27.3 kB	-11 B (-0.04%)

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[github-actions]

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Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: juanfra <juanfra@git.wordpress.org>

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[Mamaduka]

Thanks, @ciampo!

[juanfra]

Thanks for the PR @ciampo! LGTM