Font Library: Show contextual warning when deleting a font that is in use

[dhruvikpatel18]

Closes #55175

What?

Updates the delete confirmation dialog in the Font Library to show a stronger, context-aware warning when the font being deleted is currently assigned to a style target (body text, headings, elements, or blocks) in the site's global styles.

Why?

Previously, the delete confirmation dialog showed the same generic message regardless of whether the font was actively used anywhere on the site.

That gave no indication to the user that deleting the font would have a visible impact on their site — pages using that font would silently fall back to the default font with no warning. This was flagged in the FSE Outreach Program's Final Touches testing call as confusing and potentially destructive behaviour.

Testing Instructions

1. Open the Site Editor.
2. Navigate to Styles → Typography → Text and assign a custom installed font (e.g. "AR One Sans") as the font family. Save the changes.
3. Open Fonts → Library (the font library modal).
4. Select the font you just assigned (e.g. "AR One Sans").
5. Click the Delete button.
6. Confirm the dialog now shows the in-use warning message mentioning the font is currently used in site styles and that the site will fall back to the default font.
7. Cancel the dialog. Now go back to Styles → Typography → Text and remove the font assignment (set it back to Default). Save.
8. Open the font library again, select the same font, and click Delete.
9. Confirm the dialog now shows the original generic confirmation message (no in-use warning).
10. Verify deleting in both cases still works correctly, the font is removed and the success notice appears.

Screenshots or screencast

Screen.Recording.2026-05-11.at.3.57.09.PM.mov

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Co-authored-by: dhruvikpatel18 <dhruvik18@git.wordpress.org>
Co-authored-by: annezazu <annezazu@git.wordpress.org>

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