fix: keep the submit button from growing while the form is submitting

[johansolve]

Closes #2986.

Summary

The loading spinner is appended as a <span class="spinner"> inside the submit button, and it carried padding: 2rem. The button therefore grew — mostly in height — for as long as the submission was in flight, then shrank back. On a centered button the surrounding layout moves with it.

The ring itself is drawn by :before with position: absolute, its own width/height and negative margins for centering, so the span never needed a box of its own. It is now sized to the ring instead.

Second change on the same element: the ring's colour defaulted to white, which assumes a dark button. --submit-color is only set when the user picks a submit text colour in the block, so out of the box the spinner is white — invisible on any light or brightly coloured button. Since the span sits inside the button, currentColor inherits the button's own text colour and works on both light and dark.

Screenshots

Measured on a 46px-tall button with a lime background and dark text:

	Button height, idle	Button height, submitting	Spinner visible
Before	46px	grows	no (white on lime)
After	46px	46px	yes

Test instructions

1. Add a Form block to a page and give the submit button a visible padding and a light background colour.
2. Submit the form on the frontend and watch the button while the request is pending.
3. Before: the button grows and the spinner cannot be seen. After: the button keeps its height and the spinner is drawn in the button's text colour.
4. Check a dark button too, where the ring previously showed: it still does, now inheriting the text colour rather than being hard-coded white.

The change is two declarations in style.scss. I verified the compiled output and the resulting geometry in a browser against a real form, but I could not run the E2E suite locally — it needs wp-env and Docker was unavailable on this machine.

If you would rather keep white as the fallback for backwards compatibility, dropping the padding alone still fixes the reported bug — happy to trim the PR to that.

Checklist before the final review

• Included E2E or unit tests for the changes in this PR.
• Visual elements are not affected by independent changes.
• It is at least compatible with the minimum WordPress version.
• It loads additional script in frontend only if it is required.
• Does not impact the Core Web Vitals.
• In case of deprecation, old blocks are safely migrated.
• It is usable in Widgets and FSE.
• Copy/Paste is working if the attributes are modified.

This is a visual change by definition — the spinner is the visual element in question, and the button around it stops moving. No tests included: asserting "the button does not change height mid-request" needs an intercepted, held-open submission in E2E. I can write that if you want it.

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Created with help of Claude Code.