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/components/AppLayout/index.tsx

@@ -1,5 +1,5 @@
 import { useState } from 'react';
-import { Layout, Menu } from 'antd';
+import { Layout, Menu, theme } from 'antd';
 import { CloudServerOutlined, BarChartOutlined, SettingOutlined } from '@ant-design/icons';
 import { Outlet, useLocation, useNavigate } from 'react-router';
 import { useTheme } from '@/contexts/ThemeContext';
@@ -17,6 +17,7 @@ export function AppLayout() {
   const { effectiveThemeId } = useTheme();
   const isDark = effectiveThemeId === 'dark';
   const [collapsed, setCollapsed] = useState(false);
+  const { token } = theme.useToken();
 
   const getPageTitle = () => {
     if (location.pathname === '/providers') return '供应商管理';
@@ -49,7 +50,7 @@ export function AppLayout() {
               display: 'flex',
               alignItems: 'center',
               justifyContent: 'center',
-              color: isDark ? '#fff' : 'rgba(0, 0, 0, 0.88)',
+              color: token.colorText,
               fontSize: collapsed ? '1rem' : '1.25rem',
               fontWeight: 600,
               transition: 'all 0.2s',
@@ -72,10 +73,10 @@ export function AppLayout() {
         <Layout.Header
           style={{
             padding: '0 2rem',
-            background: effectiveThemeId === 'dark' ? '#141414' : '#fff',
+            background: token.colorBgContainer,
             display: 'flex',
             alignItems: 'center',
-            borderBottom: effectiveThemeId === 'dark' ? '1px solid #303030' : '1px solid #f0f0f0',
+            borderBottom: `1px solid ${token.colorBorderSecondary}`,
           }}
         >
           <h1 style={{ margin: 0, fontSize: '1.25rem' }}>{getPageTitle()}</h1>