feat(dashboard): add group usage distribution chart to usage page

Add a doughnut chart showing usage statistics broken down by group on
the admin usage records page. The chart appears alongside the existing
model distribution chart (2-column grid), with the token usage trend
chart moved to a separate full-width row below.

Changes:
- backend/pkg/usagestats: add GroupStat type
- backend/service: add GetGroupStatsWithFilters interface method and implementation
- backend/repository: implement GetGroupStatsWithFilters with LEFT JOIN groups
- backend/handler: add GetGroupStats handler with full filter support
- backend/routes: register GET /admin/dashboard/groups route
- backend/tests: add GetGroupStatsWithFilters stubs to contract/sora tests
- frontend/types: add GroupStat interface
- frontend/api: add getGroupStats API function and types
- frontend/components: add GroupDistributionChart.vue doughnut chart
- frontend/views: update UsageView layout and load group stats in parallel
- frontend/i18n: add groupDistribution, group, noGroup keys (zh + en)

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"`