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Click on a group name, model name, or endpoint name in the distribution
tables to expand and show per-user usage breakdown (requests, tokens,
actual cost, standard cost).

Backend: new GET /admin/dashboard/user-breakdown API with group_id,
model, endpoint, endpoint_type filters.
Frontend: clickable rows with expand/collapse sub-table in all three
distribution charts.

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)

- 统一移除语句末尾分号，规范代码格式
- 优化组件类型定义和 props 声明
- 改进组件文档和示例代码
- 提升代码可读性和一致性