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)

frontend/src/api/admin/dashboard.ts

@@ -8,6 +8,7 @@ import type {
   DashboardStats,
   TrendDataPoint,
   ModelStat,
+  GroupStat,
   ApiKeyUsageTrendPoint,
   UserUsageTrendPoint,
   UsageRequestType
@@ -101,6 +102,34 @@ export async function getModelStats(params?: ModelStatsParams): Promise<ModelSta
   return data
 }
 
+export interface GroupStatsParams {
+  start_date?: string
+  end_date?: string
+  user_id?: number
+  api_key_id?: number
+  account_id?: number
+  group_id?: number
+  request_type?: UsageRequestType
+  stream?: boolean
+  billing_type?: number | null
+}
+
+export interface GroupStatsResponse {
+  groups: GroupStat[]
+  start_date: string
+  end_date: string
+}
+
+/**
+ * Get group usage statistics
+ * @param params - Query parameters for filtering
+ * @returns Group usage statistics
+ */
+export async function getGroupStats(params?: GroupStatsParams): Promise<GroupStatsResponse> {
+  const { data } = await apiClient.get<GroupStatsResponse>('/admin/dashboard/groups', { params })
+  return data
+}
+
 export interface ApiKeyTrendParams extends TrendParams {
   limit?: number
 }
@@ -203,6 +232,7 @@ export const dashboardAPI = {
   getRealtimeMetrics,
   getUsageTrend,
   getModelStats,
+  getGroupStats,
   getApiKeyUsageTrend,
   getUserUsageTrend,
   getBatchUsersUsage,