fix: Space stacked form actions and stop Sass injecting a mid-file charset

[hellopablo]

Summary

• Adds form__actions--stacked so vertically stacked buttons get consistent gap, including when each button is wrapped in its own form.
• Disables Sass charset output so a BOM cannot land mid-file and wipe :root spacing tokens, and rebuilds nails.min.css.

Test plan

• Confirm stacked MFA/auth buttons have space between them
• Confirm --spacing-* custom properties resolve on a rebuilt stylesheet

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