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/types/index.ts

@@ -1086,6 +1086,15 @@ 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