Toolbar: remove white background from transparent site icon

[fushar]

Trac ticket: https://core.trac.wordpress.org/ticket/65974

This PR removes white background from transparent site icons. This is done so that site icons maintain their original shape.

I actually implemented the initial background color so that the site icons are consistently square, but now that the feature is live and I've seen some client sites, I think it's a mistake. In some brands, the shape of a transparent site icon gets weird with the added white rounded square.

With this change, these 4 places are now consistent:

• toolbar
• browser tabs (at least Chrome and Firefox show transparent favicons as-is)
• Settings -> General site icon preview
• Site Editor -> Identity -> Site Icon form input

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Solid site icon (no change)

Toolbar

Settings -> General

Site Editor -> Identity

Actual browser Chrome tab

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Transparent site icon

Toolbar - BEFORE vs AFTER

Settings -> General - BEFORE vs AFTER

Site Editor -> Identity

Actual browser (Chrome) tab

Use of AI Tools

Used Claude Code, Opus 5 to help generate the CSS changes.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[jasmussen]

This feels valid to me. It's good to have the square silhouette for site logos when any arbitrary image is used, but we shouldn't necessarily enforce a square shape for logos that feature transparency.

[github-actions]

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Props fushar, joen.

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