[copilot] Front end build for new topic discovery component

[Nabeel1276]

Resolves JIRA: https://bbc.atlassian.net/browse/WS-2397

Summary

• Creates a new Topic Discovery component which renders the 4 most recent articles related to a topic within each topic tag tab returned via the BFF
• Implementation is via a tabbed onward-journey module in the TopicDiscovery component which renders topic-based recommendations returned by the BFF, showing up to 4 related items per topic and supporting article, audio and video promos
• Component renders after the Article related-content section
• Adds/updates test coverage, provides supporting documentation for the new component as well as a storybook component

Code changes

• See files changed.

Testing

The component can be viewed in Storybook:
https://ws-2397-front-end-build-for-new-topic--5d28eb3fe163f6002046d6fa.chromatic.com/iframe.html?id=pages-article-page--article-page-with-topic-discovery&viewMode=story

But also feel free to run simorgh using yarn dev to view the component

Useful Links

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• Add Links to useful resources related to this PR if applicable.

[Copilot]

Pull request overview

Adds a new TopicDiscovery UI component to render topic-based related content using a tabbed interface, reusing existing curation promo/grid building blocks in Simorgh.

Changes:

• Introduces TopicDiscovery component, styles, and data/types to render a tabbed related-content section.
• Adds a ScrollableTabs subcomponent for horizontally scrollable, keyboard-navigable tabs with scroll controls.
• Adds Storybook stories and fixtures for local development/visual testing.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
src/app/components/TopicDiscovery/types.ts	Adds TypeScript types for topic discovery payloads.
src/app/components/TopicDiscovery/index.tsx	Implements the main TopicDiscovery section, tab state, and mapping items to curation summaries.
src/app/components/TopicDiscovery/index.styles.ts	Emotion styles for the TopicDiscovery section layout.
src/app/components/TopicDiscovery/index.stories.tsx	Storybook stories for default, single-topic, and empty states.
src/app/components/TopicDiscovery/fixtures.ts	Fixture data for Storybook/dev testing.
src/app/components/TopicDiscovery/ScrollableTabs/index.tsx	Implements scrollable tab UI with overflow detection and keyboard interaction.
src/app/components/TopicDiscovery/ScrollableTabs/index.styles.ts	Emotion styles for ScrollableTabs and scroll buttons.