Remove marketing soft opt in for US: Hide toggle and apply removed soft opt in (client-side logic)

[georgerichmond]

Overview

Implement the UI logic for removing the marketing soft opt in for US newsletter sign-ups. When enabled for signed out US users, the marketing toggle is hidden and the user is automatically opted into similar_guardian_products.

Acceptance Criteria

• Add a helper to check whether the US soft opt-in switch is enabled (via window.guardian.config.switches.us-signup-hide-marketing-toggle)
• Use the useCountryCode hook to determine country
• In all sign-up surfaces (SecureSignup, NewsletterSignupForm, ManyNewsletterSignUp):
  • If the switch is OFF, current behaviour (toggle visible for all users)
  • If the switch is ON and countryCode === 'US', hide the toggle and always POST marketing=true
  • All other countries: toggle visible as normal
  • No changes for signed-in users
• Treat countryCode === undefined (pending) as non-US — show toggle while resolving
• Add/update unit tests to cover US, non-US, and pending states
• Ensure that a marketingOptInHidden field is included in the sign-up payload, alongside countryCode: 'US', when the hidden soft opt-in is applied
• Analytics tracking: the marketingOptInType field sent in Ophan tracking events should reflect the soft opt-in case
  • When the toggle is hidden (US soft opt-in applied), send `marketingOptInType: 'similar-guardian-products-optin-hidden-us'
  • When the toggle is visible and the user opts in, send similar-guardian-products-optin
  • When the toggle is visible and the user opts out, send similar-guardian-products-optout
  • For signed-in users, omit marketingOptInType from the tracking payload
  • Applies to all three surfaces: SecureSignup, NewsletterSignupForm, ManyNewsletterSignUp

Additional Notes

• The switch for this functionality is currently referred to as us-signup-hide-marketing-toggle in the codebase; ensure consistent usage across code, tests, and stories.
• Parent issue: guardian/email-rendering#930