feat(usage): add group usage distribution chart alongside model distribution

- 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

frontend/src/i18n/locales/en.ts

@@ -828,9 +828,11 @@ export default {
       day: 'Day',
       hour: 'Hour',
       modelDistribution: 'Model Distribution',
+      groupDistribution: 'Group Distribution',
       tokenUsageTrend: 'Token Usage Trend',
       userUsageTrend: 'User Usage Trend (Top 12)',
       model: 'Model',
+      group: 'Group',
       requests: 'Requests',
       tokens: 'Tokens',
       actual: 'Actual',