Enable Compare Groups for mutation gene bar charts

[daksha607]

Fixes cBioPortal/cbioportal#11771

Summary

• Enables the Compare Groups option for mutation gene-specific bar charts
• Aligns mutation charts behavior with CNA gene-specific charts

Implementation details

• Adds showComparisonPageIcon to mutation gene bar plots
• Reuses existing openComparisonPage logic from ResultsViewPageStore
• Frontend-only change; no backend or API changes

Testing

• Manually verified in Results View that mutation gene bar charts now show the
  Compare Groups option and correctly navigate to the comparison page

Has tests or has a separate issue that describes the types of test that should be created.
This is a UI-only menu enablement. No existing test infrastructure covers this menu path.

Is this PR adding logic based on one or more clinical attributes?
NO. This is purely UI logic, no clinical data handling.

Checklist
I have read, understood, and followed the contributing guidelines.