Migrate GOV.UK components to USWDS

- Convert previous/next navigation to USWDS pagination component
  - Update radio buttons and checkboxes to use USWDS classes
  - Migrate guest list inputs with proper USWDS styling
  - Replace GOV.UK button patterns with USWDS button variants
  - Update form field spacing to use USWDS utilities
  - Scope fieldset legend styles to specific form contexts
  - Update visual regression test references for new components

app/assets/javascripts/main.js

@@ -7,9 +7,6 @@ $(() => GOVUK.modules.start());
 
 $(() => $('.error-message, .usa-error-message').eq(0).parent('label').next('input').trigger('focus'));
 
-$(() => $('.govuk-header__container').on('click', function() {
-  $(this).css('border-color', '#005ea5');
-}));
 
 // Applies our expanded focus style to the siblings of links when that link is wrapped in a heading.
 //