feat(dashboard): add per-user drill-down for group, model, and endpoint distributions

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.

frontend/src/views/admin/DashboardView.vue

@@ -246,6 +246,8 @@
               :loading="chartsLoading"
               :ranking-loading="rankingLoading"
               :ranking-error="rankingError"
+              :start-date="startDate"
+              :end-date="endDate"
               @ranking-click="goToUserUsage"
             />
             <TokenUsageTrend :trend-data="trendData" :loading="chartsLoading" />