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

[hellopablo]

Summary

• Mirrors form__actions--stacked and the Sass charset safeguard from feature/pre-new-admin onto develop.
• Resolves against the current develop Webpack configuration and rebuilds with freshly installed Yarn dependencies.
• The generated CSS was already byte-equivalent on develop, so only the SCSS source and Webpack configuration differ.

Original feature/pre-new-admin PR: #563.

Test plan

• yarn install && yarn build succeeds (only existing Sass @import deprecation warnings)
• PHPUnit: 316 tests, 520 assertions (2 deprecations)
• Confirm stacked MFA actions retain spacing

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