Fixed delayed appearance of heading separator in SubNav

[rezrah]

Summary

Follow up to #824

The heading separator in SubNav now uses CSS to handle visibility instead of JS. This fixes the delayed loading issue observed in dotcom, which doesn't happen in Storybook.

List of notable changes:

• Removed the conditional JS-based logic for making the separator appear
• Added new CSS class to hide the separator when active labels aren't needed. This replicates the same behaviour that was previously applied in JS.

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.2024-12-17.at.12.04.35.mov	Screen.Recording.2024-12-17.at.12.05.51.mov

[changeset-bot]

🦋 Changeset detected

Latest commit: 72e82db

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.