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

frontend/src/i18n/locales/en.ts

@@ -732,6 +732,7 @@ export default {
     totalCost: 'Total Cost',
     standardCost: 'Standard',
     actualCost: 'Actual',
+    accountCost: 'Cost',
     userBilled: 'User billed',
     accountBilled: 'Account billed',
     accountMultiplier: 'Account rate',
@@ -1039,6 +1040,7 @@ export default {
       tokens: 'Tokens',
       actual: 'Actual',
       standard: 'Standard',
+      accountCost: 'Cost',
       noDataAvailable: 'No data available',
       recentUsage: 'Recent Usage',
       viewModelDistribution: 'Model Distribution',