fix(admin/usage): 恢复成本 Tooltip 明细并优化账号筛选

问题修复:
- 恢复 Cost Tooltip 的成本分项明细 (input_cost, output_cost, cache 成本)
- 修复 Token Tooltip 双分隔线显示问题
- 修复 Tooltip 翻译键缺失问题，新增 costDetails/tokenDetails
- 恢复 Excel 导出格式化 (aoa_to_sheet + 翻译列头)

功能优化:
- 账号筛选从前端搜索改为后端搜索，避免一次加载 1000 条数据
- 行为与用户/API Key 筛选保持一致 (debounce + 后端分页)

frontend/src/i18n/locales/en.ts

@@ -376,6 +376,8 @@ export default {
   usage: {
     title: 'Usage Records',
     description: 'View and analyze your API usage history',
+    costDetails: 'Cost Breakdown',
+    tokenDetails: 'Token Breakdown',
     totalRequests: 'Total Requests',
     totalTokens: 'Total Tokens',
     totalCost: 'Total Cost',
@@ -1691,6 +1693,7 @@ export default {
       userFilter: 'User',
       searchUserPlaceholder: 'Search user by email...',
       searchApiKeyPlaceholder: 'Search API key by name...',
+      searchAccountPlaceholder: 'Search account by name...',
       selectedUser: 'Selected',
       user: 'User',
       account: 'Account',

frontend/src/i18n/locales/zh.ts

@@ -373,6 +373,8 @@ export default {
   usage: {
     title: '使用记录',
     description: '查看和分析您的 API 使用历史',
+    costDetails: '成本明细',
+    tokenDetails: 'Token 明细',
     totalRequests: '总请求数',
     totalTokens: '总 Token',
     totalCost: '总消费',
@@ -1836,6 +1838,7 @@ export default {
       userFilter: '用户',
       searchUserPlaceholder: '按邮箱搜索用户...',
       searchApiKeyPlaceholder: '按名称搜索 API 密钥...',
+      searchAccountPlaceholder: '按名称搜索账号...',
       selectedUser: '已选择',
       user: '用户',
       account: '账户',