The expanded user breakdown rows already align with the parent table
columns (Requests, Token, Actual, Standard), so the repeated sub-header
wastes vertical space. Remove the <thead> from UserBreakdownSubTable.
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.
