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

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Latest commit: 41129e7

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