CI/CD Group Analytics: Add sub-group, date range, and default branch filters
## Summary
Add the top-of-page filter bar to the CI/CD Group Analytics dashboard. This is gated behind the `ci_group_analytics` feature flag.
## Filters to implement
1. **Sub-group filter** — allows users to select:
- No filter (Show all projects)
- Direct projects in the group only
- One or more specific subgroups
2. **Date range filter** — allows users to select the time range:
- Last 24 hours
- Last week
- Last 30 days
3. **Default branch checkbox** — filters out pipelines not on the default branch
## Acceptance Criteria
- All three filters are rendered at the top of the dashboard page
- Selecting filter values updates the dashboard state (even if panels are not yet implemented)
- Filters are gated behind the `ci_group_analytics` feature flag
issue