fix(groups): 用户分组不下发内部路由信息

- 普通用户 Group DTO 移除 model_routing/account_count/account_groups，避免泄露内部路由与账号信息\n- 新增 AdminGroup DTO，并仅在管理员分组接口返回完整字段\n- 前端拆分 Group/AdminGroup，管理端页面与 API 使用 AdminGroup\n- 增加 /api/v1/groups/available 契约测试，防止回归

frontend/src/views/admin/AccountsView.vue

@@ -187,14 +187,14 @@ import AccountCapacityCell from '@/components/account/AccountCapacityCell.vue'
 import PlatformTypeBadge from '@/components/common/PlatformTypeBadge.vue'
 import Icon from '@/components/icons/Icon.vue'
 import { formatDateTime, formatRelativeTime } from '@/utils/format'
-import type { Account, Proxy, Group } from '@/types'
+import type { Account, Proxy, AdminGroup } from '@/types'
 
 const { t } = useI18n()
 const appStore = useAppStore()
 const authStore = useAuthStore()
 
 const proxies = ref<Proxy[]>([])
-const groups = ref<Group[]>([])
+const groups = ref<AdminGroup[]>([])
 const selIds = ref<number[]>([])
 const showCreate = ref(false)
 const showEdit = ref(false)