feat(usage): add account cost display to admin dashboard and usage pages

- Add account_cost column to dashboard aggregation tables (migration 107)
- DashboardStats: add TotalAccountCost/TodayAccountCost fields
- ModelStat/GroupStat: add AccountCost field with SQL aggregation
- GetStatsWithFilters: always return TotalAccountCost (remove accountID filter)
- Dashboard Token cards: show user(green)/cost(orange)/standard(gray)
- Usage stats card: show account cost and standard below main value
- Model/Group distribution tables: add orange cost column

backend/migrations/107_add_account_cost_to_dashboard_tables.sql

@@ -0,0 +1,5 @@
+-- Add account_cost column to dashboard aggregation tables for admin dashboard display.
+-- account_cost = SUM(COALESCE(account_stats_cost, total_cost) * COALESCE(account_rate_multiplier, 1))
+
+ALTER TABLE usage_dashboard_hourly ADD COLUMN IF NOT EXISTS account_cost DECIMAL(20, 10) NOT NULL DEFAULT 0;
+ALTER TABLE usage_dashboard_daily ADD COLUMN IF NOT EXISTS account_cost DECIMAL(20, 10) NOT NULL DEFAULT 0;