feat: 网盘设置加「默认账号」列, 同类型最多2个主账号

- 数据库 migration: cloud_configs 加 is_primary 字段
- 后端: togglePrimary API (PUT /admin/cloud-configs/:id/primary)
- 后端: getAndValidateCredential 优先选 is_primary 账号
- 前端: CloudConfig.vue 转存启用后加「默认账号」开关列
- 前端: api/index.ts 加 setPrimary 方法

packages/frontend/src/api/index.ts

@@ -267,6 +267,14 @@ export async function deleteCloudConfig(
   await api.delete(`/admin/cloud-configs/${id}`)
 }
 
+export async function setPrimary(
+  id: number,
+  primary: boolean
+): Promise<any> {
+  const { data } = await api.put(`/admin/cloud-configs/${id}/primary`, { primary })
+  return data
+}
+
 export async function getStats(days?: number): Promise<StatsData> {
   const params: Record<string, number> = {}
   if (days) params.days = days