perf: 优化体积，highlight.js按需导入与i18n按需加载 (#696)

* perf: 优化打包体积，highlight.js按需导入与i18n按需加载

1. highlight.js: 从全量导入改为 core + 注册27种常用语言，减少约500~800KB
2. i18n: 只同步加载en语言包，其他8种语言改为异步加载，首屏减少约350~400KB
3. 使用vue-i18n的setLocaleMessage API动态注册语言包
4. 新增switchLocale函数统一处理语言切换
5. 同步更新相关测试文件的mock路径和API适配

* 修复类型断言

packages/client/src/i18n/messages.ts

@@ -1,26 +1,9 @@
-import de from './locales/de'
 import en from './locales/en'
-import es from './locales/es'
-import fr from './locales/fr'
-import ja from './locales/ja'
-import ko from './locales/ko'
-import pt from './locales/pt'
-import zh from './locales/zh'
-import zhTW from './locales/zh-TW'
 
-export type LocaleMessages = Record<string, unknown>
+export type LocaleMessages = Record<string, any>
 
-export const rawMessages = {
-  'en': en,
-  'zh': zh,
-  'zh-TW': zhTW,
-  'ja': ja,
-  'ko': ko,
-  'fr': fr,
-  'es': es,
-  'de': de,
-  'pt': pt,
-} satisfies Record<string, LocaleMessages>
+export const supportedLocales = ['en', 'zh', 'zh-TW', 'ja', 'ko', 'fr', 'es', 'de', 'pt'] as const
+export type SupportedLocale = (typeof supportedLocales)[number]
 
 function isPlainObject(value: unknown): value is LocaleMessages {
   return !!value && typeof value === 'object' && !Array.isArray(value)
@@ -42,11 +25,25 @@ export function mergeMessagesWithFallback(
   return merged
 }
 
-export const messages = Object.fromEntries(
-  Object.entries(rawMessages).map(([locale, localeMessages]) => [
-    locale,
-    locale === 'en'
-      ? localeMessages
-      : mergeMessagesWithFallback(en, localeMessages),
-  ]),
-) as typeof rawMessages
+const localeLoaders: Record<string, () => Promise<{ default: LocaleMessages }>> = {
+  'zh': () => import('./locales/zh'),
+  'zh-TW': () => import('./locales/zh-TW'),
+  'ja': () => import('./locales/ja'),
+  'ko': () => import('./locales/ko'),
+  'fr': () => import('./locales/fr'),
+  'es': () => import('./locales/es'),
+  'de': () => import('./locales/de'),
+  'pt': () => import('./locales/pt'),
+}
+
+export { en }
+
+export async function loadLocale(locale: string): Promise<LocaleMessages | null> {
+  if (locale === 'en') return en
+
+  const loader = localeLoaders[locale]
+  if (!loader) return null
+
+  const mod = await loader()
+  return mergeMessagesWithFallback(en, mod.default)
+}