Subscription webview go back action

[marcosholgado]

Task/Issue URL: https://app.asana.com/1/137249556945/project/488551667048375/task/1211073283656160?focus=true

Description

This PR changes the way navigating back works letting the FE decide in some cases when it can and cannot happen

Steps to test this PR

• Apply the staging patch and VPN into US
• Go to https://duckduckgo.com/pro?featurePage=stripe
• Complete the purchase using the test cards for Stripe
• On the welcome screen, the icon should be a cross rather than an arrow.
• Pressing either the cross or the back button should take you back to settings (doing this on develop would take you back to Stripe)
• Now that you are subscribe go to settings and click on "Need help with your subscription"
• On that webview navigate to some links, see how the cross turns into an arrow and if you now press it or use the back button you navigate back rather than close the screen.

[marcosholgado]

• Subscription webview go back action #6752 👈 (View in Graphite)
• develop

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[nalcalag]

LGTM! However, I noticed a few scenarios where the “X” appears instead of the arrow, and I’m not sure if this is intended:

1. Paywall screen –> On the second step, it still shows the cross instead of the arrow.
   https://github.com/user-attachments/assets/2e7d6084-b03b-4978-a60b-aa08068d439f

2. Add subscription with email –> In some steps, the cross is replaced by the arrow, but not consistently.
   https://github.com/user-attachments/assets/5c9aae0d-09b5-4829-9bac-602d91619870
   I assume this happens because we’re listening to a query parameter in the URL, so it’s not really coming from our code, but just double-checking to confirm.