fix(grid): keep group headers sticky with sticky columns

[tatakaisun]

Summary

Fixes grouped grid headers so custom group headers follow sticky start columns during horizontal scrolling.

Changes

• Add a Storybook reproduction for grouped rows with a custom group header and sticky columns
• Keep group row cells full-width while making the inner group header sticky
• Add a regression test for sticky group headers

Testing

• git diff --check

Known Issues

• Local dependency install requires FONT_AWESOME_AUTH_TOKEN, so targeted vitest and Storybook verification could not be run in this environment.

Closes #3367

[changeset-bot]

🦋 Changeset detected

Latest commit: 2f235af

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

This PR includes changesets to release 1 package

Name	Type
@sl-design-system/grid	Patch

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[michal-sanoma]

Please resolve conflicts