Sign-in gate v2

[juabara]

What does this change?

This PR implements the new Sign-in Gate V2 - Redesign. The new sign-in gate now displays in a modal dialog, with two layout options: dismissable and non-dismissable. The design is responsive, adapting to mobile, phablet, and desktop breakpoints.

• The implementation follows the new designs as closely as possible, but some elements (mainly CTAs) reuse V1 styles due to our dependency on the I&T team for iframe support ([see here](feat(iframe-integration): add comprehensive iframe integration support gateway#3251)).

• For testing and previewing purposes:

  • A prop was added to allow selecting V2 in sign-in components and Storybook.
  • You can also view the new gate by adding auxia_gate_version=v2 as a query string parameter.

• Note: The prop for version selection is temporary and will likely be removed once integration details with Auxia’s API are finalised.

• Important: There is a known logic conflict with RR banners—please do not merge until this is resolved.

Why?

• To deliver the updated, more flexible sign-in gate experience as per new design requirements.
• The responsive modal provides a better user experience across all device types.
• Adding temporary toggles and Storybook visibility helps with QA, review, and the ongoing collaboration with Auxia and I&T teams.

Screenshots

[github-actions]

Deploy build 44511 of Mobile::mobile-apps-rendering to CODE

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• Deploy build 44511 of Mobile::mobile-apps-rendering to CODE
• Deploy parts of build 44511 to CODE by previewing it first
• What's on CODE right now?

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From guardian/actions-riff-raff.

[github-actions]

Deploy build 20284 of dotcom:rendering-all to CODE

All deployment options

• Deploy build 20284 of dotcom:rendering-all to CODE
• Deploy parts of build 20284 to CODE by previewing it first
• What's on CODE right now?

---

From guardian/actions-riff-raff.

[shtukas]

Looks good to me. Happy to +1 But need @tomrf1 's approval as well.

[github-actions]

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