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

backend/internal/pkg/usagestats/usage_log_types.go

@@ -78,6 +78,16 @@ type ModelStat struct {
 	ActualCost   float64 `json:"actual_cost"` // 实际扣除
 }
 
+// GroupStat represents usage statistics for a single group
+type GroupStat struct {
+	GroupID   int64   `json:"group_id"`
+	GroupName string  `json:"group_name"`
+	Requests  int64   `json:"requests"`
+	TotalTokens int64 `json:"total_tokens"`
+	Cost      float64 `json:"cost"`        // 标准计费
+	ActualCost float64 `json:"actual_cost"` // 实际扣除
+}
+
 // UserUsageTrendPoint represents user usage trend data point
 type UserUsageTrendPoint struct {
 	Date       string  `json:"date"`