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/server/api_contract_test.go

@@ -1572,6 +1572,10 @@ func (r *stubUsageLogRepo) GetModelStatsWithFilters(ctx context.Context, startTi
 	return nil, errors.New("not implemented")
 }
 
+func (r *stubUsageLogRepo) GetGroupStatsWithFilters(ctx context.Context, startTime, endTime time.Time, userID, apiKeyID, accountID, groupID int64, requestType *int16, stream *bool, billingType *int8) ([]usagestats.GroupStat, error) {
+	return nil, errors.New("not implemented")
+}
+
 func (r *stubUsageLogRepo) GetAPIKeyUsageTrend(ctx context.Context, startTime, endTime time.Time, granularity string, limit int) ([]usagestats.APIKeyUsageTrendPoint, error) {
 	return nil, errors.New("not implemented")
 }

backend/internal/server/routes/admin.go

@@ -184,6 +184,7 @@ func registerDashboardRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 		dashboard.GET("/realtime", h.Admin.Dashboard.GetRealtimeMetrics)
 		dashboard.GET("/trend", h.Admin.Dashboard.GetUsageTrend)
 		dashboard.GET("/models", h.Admin.Dashboard.GetModelStats)
+		dashboard.GET("/groups", h.Admin.Dashboard.GetGroupStats)
 		dashboard.GET("/api-keys-trend", h.Admin.Dashboard.GetAPIKeyUsageTrend)
 		dashboard.GET("/users-trend", h.Admin.Dashboard.GetUserUsageTrend)
 		dashboard.POST("/users-usage", h.Admin.Dashboard.GetBatchUsersUsage)