feat(docs): remember language preference

Persist the active documentation locale and restore it when users revisit the root URL. Open the VitePress development site automatically on startup.

docs/.vitepress/theme/index.ts

@@ -1,4 +1,63 @@
+import type { Theme } from 'vitepress'
 import DefaultTheme from 'vitepress/theme'
 import './style.css'
 
-export default DefaultTheme
+type DocsLocale = 'en' | 'cn' | 'tw'
+
+const LANGUAGE_PREFERENCE_KEY = 'chatlab-docs-language'
+
+function getLocaleFromPath(path: string): DocsLocale {
+  if (path === '/cn' || path.startsWith('/cn/')) return 'cn'
+  if (path === '/tw' || path.startsWith('/tw/')) return 'tw'
+  return 'en'
+}
+
+function getBrowserLocale(): DocsLocale {
+  const languages = navigator.languages.length > 0 ? navigator.languages : [navigator.language]
+
+  for (const language of languages) {
+    const normalized = language.toLowerCase()
+    if (/^zh-(tw|hk|mo|hant)/.test(normalized)) return 'tw'
+    if (normalized.startsWith('zh')) return 'cn'
+  }
+
+  return 'en'
+}
+
+function readPreferredLocale(): DocsLocale | null {
+  try {
+    const locale = localStorage.getItem(LANGUAGE_PREFERENCE_KEY)
+    return locale === 'en' || locale === 'cn' || locale === 'tw' ? locale : null
+  } catch {
+    return null
+  }
+}
+
+function savePreferredLocale(locale: DocsLocale): void {
+  try {
+    localStorage.setItem(LANGUAGE_PREFERENCE_KEY, locale)
+  } catch {
+    // Language preference persistence is optional.
+  }
+}
+
+export default {
+  extends: DefaultTheme,
+  enhanceApp({ router }) {
+    if (typeof window === 'undefined') return
+
+    const initialPath = window.location.pathname
+    if (initialPath === '/') {
+      const preferredLocale = readPreferredLocale() ?? getBrowserLocale()
+      if (preferredLocale !== 'en') {
+        window.location.replace(`/${preferredLocale}/${window.location.search}${window.location.hash}`)
+        return
+      }
+    }
+
+    savePreferredLocale(getLocaleFromPath(initialPath))
+    router.onAfterRouteChange = (to) => {
+      savePreferredLocale(getLocaleFromPath(to))
+    }
+  },
+} satisfies Theme

docs/package.json

@@ -4,7 +4,7 @@
   "private": true,
   "type": "module",
   "scripts": {
-    "dev": "vitepress dev . --port 3120",
+    "dev": "vitepress dev . --port 3120 --open",
     "build": "vitepress build .",
     "preview": "vitepress preview ."
   },