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/types/index.ts

@@ -1072,6 +1072,15 @@ export interface ModelStat {
   actual_cost: number // 实际扣除
 }
 
+export interface GroupStat {
+  group_id: number
+  group_name: string
+  requests: number
+  total_tokens: number
+  cost: number // 标准计费
+  actual_cost: number // 实际扣除
+}
+
 export interface UserUsageTrendPoint {
   date: string
   user_id: number