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/zh.ts

@@ -736,6 +736,7 @@ export default {
     totalCost: '总消费',
     standardCost: '标准',
     actualCost: '实际',
+    accountCost: '成本',
     userBilled: '用户扣费',
     accountBilled: '账号计费',
     accountMultiplier: '账号倍率',
@@ -1026,6 +1027,7 @@ export default {
       totalCost: '总消费',
       actual: '实际',
       standard: '标准',
+      accountCost: '成本',
       todayTokens: '今日 Token',
       totalTokens: '总 Token',
       input: '输入',