Migrate radio buttons from GOV.UK to USWDS

- Replace .multiple-choice with .usa-radio classes
- Update radio templates to use USWDS patterns
- Remove GOV.UK dependencies from radio components
- Convert nested lists to flat divs with USWDS margin utilities
- Update tests to use new radio classes

app/templates/components/radios.html

@@ -40,7 +40,7 @@
      <div class="radio-select" data-module="radio-select" data-categories="{{ field.categories|join(',') }}" data-show-now-as-default="{{ show_now_as_default|string|lower }}">
        <div class="radio-select-column">
        {% for option in field %}
-         <div class="multiple-choice">
+         <div class="usa-radio">
            {{ option }}
            <label for="{{ option.id }}">
              {{ option.label.text }}