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/themes/dark.ts

@@ -1,10 +1,10 @@
 import { theme } from 'antd';
 import type { ConfigProviderProps } from 'antd';
 
-const useDarkTheme = (): ConfigProviderProps => ({
+const darkConfig: ConfigProviderProps = {
   theme: {
     algorithm: theme.darkAlgorithm,
   },
-});
+};
 
-export default useDarkTheme;
+export default darkConfig;

frontend/src/themes/default.ts

@@ -1,10 +1,10 @@
 import { theme } from 'antd';
 import type { ConfigProviderProps } from 'antd';
 
-const useDefaultTheme = (): ConfigProviderProps => ({
+const defaultConfig: ConfigProviderProps = {
   theme: {
     algorithm: theme.defaultAlgorithm,
   },
-});
+};
 
-export default useDefaultTheme;
+export default defaultConfig;

frontend/src/themes/index.ts

@@ -1,6 +1,7 @@
+import { useMemo } from 'react';
 import type { ConfigProviderProps } from 'antd';
-import useDefaultTheme from './default';
-import useDarkTheme from './dark';
+import defaultConfig from './default';
+import darkConfig from './dark';
 import useMuiTheme from './mui';
 import useShadcnTheme from './shadcn';
 import useBootstrapTheme from './bootstrap';
@@ -25,23 +26,22 @@ export const themeOptions: ThemeOption[] = [
 const themeIdSet = new Set<ThemeId>(themeOptions.map((opt) => opt.id));
 
 export function useThemeConfig(themeId: ThemeId): ConfigProviderProps {
-  const defaultConfig = useDefaultTheme();
-  const darkConfig = useDarkTheme();
   const muiConfig = useMuiTheme();
   const shadcnConfig = useShadcnTheme();
   const bootstrapConfig = useBootstrapTheme();
   const glassConfig = useGlassTheme();
 
-  const configs: Record<ThemeId, ConfigProviderProps> = {
-    default: defaultConfig,
-    dark: darkConfig,
-    mui: muiConfig,
-    shadcn: shadcnConfig,
-    bootstrap: bootstrapConfig,
-    glass: glassConfig,
-  };
-
-  return configs[themeId] ?? configs.default;
+  return useMemo(() => {
+    const configs: Record<ThemeId, ConfigProviderProps> = {
+      default: defaultConfig,
+      dark: darkConfig,
+      mui: muiConfig,
+      shadcn: shadcnConfig,
+      bootstrap: bootstrapConfig,
+      glass: glassConfig,
+    };
+    return configs[themeId] ?? configs.default;
+  }, [themeId, muiConfig, shadcnConfig, bootstrapConfig, glassConfig]);
 }
 
 export function isValidThemeId(value: string): value is ThemeId {