- Add GroupStat type to usagestats package
- Add GetGroupStatsWithFilters to UsageLogRepository interface and implement with LEFT JOIN groups
- Add GetGroupStats dashboard API endpoint (GET /admin/dashboard/groups)
- Add GroupDistributionChart.vue component mirroring ModelDistributionChart
- Rearrange UsageView layout: model + group in one row, token trend full-width below
- All filters (user, api_key, account, group, model, date range) apply to group stats