refactor: 迁移前端 antd 组件至 v6 规范 API，消除废弃用法

- 迁移静态 message 到 App.useApp() 模式，使 message 感知 ConfigProvider 上下文
- Button type/danger 迁移为 variant/color 新 API
- Space direction 迁移为 vertical 布尔属性
- Select.Option 子组件迁移为 options 属性
- AppLayout 硬编码颜色替换为 antd design token
- 优化 useThemeConfig：default/dark 改为静态导出，减少不必要的 hook 调用
- 同步更新 openspec 主规范文档

frontend/src/hooks/useModels.ts

@@ -1,5 +1,5 @@
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
-import { message } from 'antd';
+import { App } from 'antd';
 import type { CreateModelInput, UpdateModelInput } from '@/types';
 import * as api from '@/api/models';
 
@@ -17,6 +17,7 @@ export function useModels(providerId?: string) {
 
 export function useCreateModel() {
   const queryClient = useQueryClient();
+  const { message } = App.useApp();
 
   return useMutation({
     mutationFn: (input: CreateModelInput) => api.createModel(input),
@@ -32,6 +33,7 @@ export function useCreateModel() {
 
 export function useUpdateModel() {
   const queryClient = useQueryClient();
+  const { message } = App.useApp();
 
   return useMutation({
     mutationFn: ({ id, input }: { id: string; input: UpdateModelInput }) =>
@@ -48,6 +50,7 @@ export function useUpdateModel() {
 
 export function useDeleteModel() {
   const queryClient = useQueryClient();
+  const { message } = App.useApp();
 
   return useMutation({
     mutationFn: (id: string) => api.deleteModel(id),