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/i18n/locales/en.ts

@@ -408,9 +408,12 @@ export default {
     day: 'Day',
     hour: 'Hour',
     modelDistribution: 'Model Distribution',
+    groupDistribution: 'Group Usage Distribution',
     tokenUsageTrend: 'Token Usage Trend',
     noDataAvailable: 'No data available',
     model: 'Model',
+    group: 'Group',
+    noGroup: 'No Group',
     requests: 'Requests',
     tokens: 'Tokens',
     actual: 'Actual',
@@ -832,9 +835,12 @@ export default {
       day: 'Day',
       hour: 'Hour',
       modelDistribution: 'Model Distribution',
+      groupDistribution: 'Group Usage Distribution',
       tokenUsageTrend: 'Token Usage Trend',
       userUsageTrend: 'User Usage Trend (Top 12)',
       model: 'Model',
+      group: 'Group',
+      noGroup: 'No Group',
       requests: 'Requests',
       tokens: 'Tokens',
       actual: 'Actual',

frontend/src/i18n/locales/zh.ts

@@ -409,9 +409,12 @@ export default {
     day: '按天',
     hour: '按小时',
     modelDistribution: '模型分布',
+    groupDistribution: '分组使用分布',
     tokenUsageTrend: 'Token 使用趋势',
     noDataAvailable: '暂无数据',
     model: '模型',
+    group: '分组',
+    noGroup: '无分组',
     requests: '请求',
     tokens: 'Token',
     actual: '实际',
@@ -846,9 +849,12 @@ export default {
       day: '按天',
       hour: '按小时',
       modelDistribution: '模型分布',
+      groupDistribution: '分组使用分布',
       tokenUsageTrend: 'Token 使用趋势',
       noDataAvailable: '暂无数据',
       model: '模型',
+      group: '分组',
+      noGroup: '无分组',
       requests: '请求',
       tokens: 'Token',
       cache: '缓存',