[WS-2650]: Sign-in popup banner implementation

[SantaZena]

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 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

[Copilot]

Pull request overview

Implements the WS-2650 guest sign-in prompt as a modal dialog surfaced from the “Save for later” guest flow, reusing the existing Account promotional banner content while adding modal-specific layout and focus management.

Changes:

• Added AccountSignInModal (dialog + backdrop + Escape/close handling) and a useTrappedFocus hook for focus trapping.
• Extended PromotionalBanner to support a top image slot and style overrides, enabling reuse inside the modal.
• Updated the guest save button to open the modal, and added a Storybook entry to preview the modal state.

Reviewed changes

Copilot reviewed 10 out of 13 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
src/app/hooks/useTrappedFocus/index.ts	New focus-trap hook used by the modal.
src/app/components/SaveArticleButton/SaveArticleButtonGuest/index.tsx	Opens the sign-in modal for signed-out users.
src/app/components/PromotionalBanner/index.types.ts	Adds topImage and styleOverrides types.
src/app/components/PromotionalBanner/index.tsx	Renders topImage and applies style overrides.
src/app/components/PromotionalBanner/index.test.tsx	Adds test coverage for topImage placement.
src/app/components/Account/SignInPromotionalBanner/index.tsx	New banner composition for modal sign-in prompt.
src/app/components/Account/SignInPromotionalBanner/index.styles.tsx	Styles and responsive imagery for the modal banner.
src/app/components/Account/AccountSignInModal/index.tsx	New modal wrapper with inert/background handling and focus trap usage.
src/app/components/Account/AccountSignInModal/index.styles.tsx	Modal/backdrop styling and responsive sizing.
src/app/components/Account/AccountPromotionalBanner/index.stories.tsx	Adds Storybook story for signed-out modal state.