WS-3018: Enable background images on /send pages

[HarveyPeachey]

Resolves JIRA: WS-3018

Summary

Enable support for background images on World Service /send pages so campaign-configured forms can match the News uploader experience.

Code changes

• Added background image support on /send page layout when campaign config provides a page background template URL.
• Implemented recipe substitution to use 1280xn image variants and generate modern image-set background values.
• Kept fallback behaviour for pages without a configured image so existing gradient experience remains.
• Added/updated tests for background rendering behaviour and fallback behaviour.

Testing

1. Run unit tests for UGC send page layout and confirm background-related assertions pass.
2. Open a /send page with campaign background data and verify image background is rendered on larger breakpoints.
3. Open a /send page without campaign background data and verify fallback gradient is used.
4. Verify fixed background attachment is only applied when an image background is present.

Useful Links

• Coding Standards: https://github.com/bbc/simorgh/blob/latest/docs/Coding-Standards/README.md
• Repository use guidelines: https://github.com/bbc/simorgh-infrastructure/blob/latest/documentation/repository-guidelines.md

[Copilot]

Pull request overview

Enables campaign-configured background images on World Service /send pages by extending the UGC send page layout to render an optional background image (with a gradient fallback) and adding test coverage for the fallback scenario.

Changes:

• Add optional pageBackgroundTemplateUrl support to /send page settings and compute a background style from it (including 1280xn recipe substitution).
• Update Emotion styles to accept a dynamic background value and conditionally apply background-attachment: fixed.
• Add a unit test for the “no background URL” fallback behaviour.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
ws-nextjs-app/pages/[service]/send/[id]/UGCPageLayout.tsx	Computes background (image-set or gradient fallback) and conditionally applies fixed attachment.
ws-nextjs-app/pages/[service]/send/[id]/styles.ts	Refactors background styling to accept a dynamic background value and adds a separate fixed-attachment style.
ws-nextjs-app/pages/[service]/send/[id]/types.ts	Extends settings type with optional pageBackgroundTemplateUrl.
ws-nextjs-app/pages/[service]/send/[id]/UGCPageLayout.test.tsx	Adds assertions for gradient fallback when no background URL is provided.