feat(admin): 新增 CRS 同步预览和账号选择功能

- 后端新增 PreviewFromCRS 接口，允许用户先预览 CRS 中的账号
- 后端支持在同步时选择特定账号，不选中的账号将被跳过
- 前端重构 SyncFromCrsModal 为三步向导：输入凭据 → 预览账号 → 执行同步
- 改进表单无障碍性：添加 for/id 关联和 required 属性
- 修复 Back 按钮返回时的状态清理
- 新增 buildSelectedSet 和 shouldCreateAccount 的单元测试
- 完整的向后兼容性：旧客户端不发送 selected_account_ids 时行为不变

frontend/src/i18n/locales/en.ts

@@ -1309,10 +1309,23 @@ export default {
       syncResult: 'Sync Result',
       syncResultSummary: 'Created {created}, updated {updated}, skipped {skipped}, failed {failed}',
       syncErrors: 'Errors / Skipped Details',
-      syncCompleted: 'Sync completed: created {created}, updated {updated}',
+      syncCompleted: 'Sync completed: created {created}, updated {updated}, skipped {skipped}',
       syncCompletedWithErrors:
-        'Sync completed with errors: failed {failed} (created {created}, updated {updated})',
+        'Sync completed with errors: failed {failed} (created {created}, updated {updated}, skipped {skipped})',
       syncFailed: 'Sync failed',
+      crsPreview: 'Preview',
+      crsPreviewing: 'Previewing...',
+      crsPreviewFailed: 'Preview failed',
+      crsExistingAccounts: 'Existing accounts (will be updated)',
+      crsNewAccounts: 'New accounts (select to sync)',
+      crsSelectAll: 'Select all',
+      crsSelectNone: 'Select none',
+      crsNoNewAccounts: 'All CRS accounts are already synced.',
+      crsWillUpdate: 'Will update {count} existing accounts.',
+      crsSelectedCount: '{count} new accounts selected',
+      crsUpdateBehaviorNote:
+        'Existing accounts only sync fields returned by CRS; missing fields keep their current values. Credentials are merged by key — keys not returned by CRS are preserved. Proxies are kept when "Sync proxies" is unchecked.',
+      crsBack: 'Back',
       editAccount: 'Edit Account',
       deleteAccount: 'Delete Account',
       searchAccounts: 'Search accounts...',