Resolves JIRA: https://bbc.atlassian.net/browse/WS-2650

Summary

Adds a sign-in popup modal that surfaces the account promotional banner in a focus-trapped, dismissible dialog with responsive imagery.

Code changes

• Add AccountSignInModal component with backdrop, focus trapping and Escape/click-to-close handling.
• Add useTrappedFocus hook to keep keyboard focus within the modal.
• Add a styleOverrides prop to PromotionalBanner so consumers can override slot styles, replacing the promo-banner-* class hooks.
• Forward style overrides through AccountPromotionalBanner and render breakpoint-specific sign-in images via CSS variables.
• Add responsive sign-in images (mobile/tablet/desktop) and remove the redundant globe images.

Testing

1. Run yarn dev and open a hindi article (e.g. /hindi/articles/c0469479x9xo) (The user should not be signed in).
2. Trigger the sign-in modal by clicking on "Save for later" and confirm it opens centred over a dimmed backdrop.
3. Tab through the modal and confirm focus stays trapped inside it.
4. Press Escape, click the backdrop, and click the close button — each should dismiss the modal.
5. Resize across mobile, tablet and desktop breakpoints and confirm the correct sign-in image renders.

Useful Links

Figma design: https://www.figma.com/design/8xqdz6WAYt2QpcLguVBCFz/WS-Sign-In-prompt?node-id=117-4210&p=f&t=bLX9drCEGAV2HoZj-0