Fix RiverStoryScroll disabled bug on narrow viewports

[rezrah]

Summary

Fixes a bug where RiverStoryScroll wasn't respecting the disabled prop on narrow viewports.

🔗 New Storybook example

Closes #881

List of notable changes:

• Prevent relevant styles from rendering when they shouldn't

What should reviewers focus on?

• Review the new story

Contributor checklist:

• All new and existing CI checks pass
• Tests prove that the feature works and covers both happy and unhappy paths
• Any drop in coverage, breaking changes or regressions have been documented above
• UI Changes contain new visual snapshots (generated by adding update snapshots label to the PR)
• All developer debugging and non-functional logging has been removed
• Related issues have been referenced in the PR description

Reviewer checklist:

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change

Screenshots:

Please try to provide before and after screenshots or videos

Before	After
Screen.Recording.2025-02-03.at.11.52.10.mov	Screen.Recording.2025-02-03.at.11.52.30.mov

[changeset-bot]

🦋 Changeset detected

Latest commit: 1ab5f49

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@primer/react-brand	Patch
@primer/brand-primitives	Patch
@primer/brand-e2e	Patch
@primer/brand-fonts	Patch
@primer/brand-config	Patch
@primer/brand-storybook	Patch

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[github-actions]

🟢 No design token changes found

[github-actions]

🟢 No visual differences found

Our visual comparison tests did not find any differences in the UI.