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/api/admin/accounts.ts

@@ -327,11 +327,34 @@ export async function getAvailableModels(id: number): Promise<ClaudeModel[]> {
   return data
 }
 
+export interface CRSPreviewAccount {
+  crs_account_id: string
+  kind: string
+  name: string
+  platform: string
+  type: string
+}
+
+export interface PreviewFromCRSResult {
+  new_accounts: CRSPreviewAccount[]
+  existing_accounts: CRSPreviewAccount[]
+}
+
+export async function previewFromCrs(params: {
+  base_url: string
+  username: string
+  password: string
+}): Promise<PreviewFromCRSResult> {
+  const { data } = await apiClient.post<PreviewFromCRSResult>('/admin/accounts/sync/crs/preview', params)
+  return data
+}
+
 export async function syncFromCrs(params: {
   base_url: string
   username: string
   password: string
   sync_proxies?: boolean
+  selected_account_ids?: string[]
 }): Promise<{
   created: number
   updated: number
@@ -345,7 +368,19 @@ export async function syncFromCrs(params: {
     error?: string
   }>
 }> {
-  const { data } = await apiClient.post('/admin/accounts/sync/crs', params)
+  const { data } = await apiClient.post<{
+    created: number
+    updated: number
+    skipped: number
+    failed: number
+    items: Array<{
+      crs_account_id: string
+      kind: string
+      name: string
+      action: string
+      error?: string
+    }>
+  }>('/admin/accounts/sync/crs', params)
   return data
 }
 
@@ -442,6 +477,7 @@ export const accountsAPI = {
   batchCreate,
   batchUpdateCredentials,
   bulkUpdate,
+  previewFromCrs,
   syncFromCrs,
   exportData,
   importData,