CI/CD Group Analytics: Create empty dashboard page behind `ci_group_analytics` feature flag
## Summary
Create a new page for the CI/CD Group Analytics dashboard, gated behind the `ci_group_analytics` feature flag. The page should render an empty shell (no content yet) that will be progressively filled in by subsequent issues.
## Acceptance Criteria
- A new route exists for the group-level CI/CD analytics dashboard
- The page is only accessible when the `ci_group_analytics` feature flag is enabled
- The page renders without errors (empty state is acceptable)
- The feature flag `ci_group_analytics` is documented and registered
## Notes
- This is the foundational scaffold for the entire CI/CD Group Analytics feature
- All subsequent FE and BE work for this feature should also be gated behind `ci_group_analytics`
issue