Add root page with language detection and redirect

Co-authored-by: huangyiirene <7665279+huangyiirene@users.noreply.github.com>

Author: Copilot

packages/site/app/page.tsx

@@ -0,0 +1,63 @@
+import { redirect } from 'next/navigation';
+import { headers } from 'next/headers';
+import { siteConfig } from '@/lib/site-config';
+
+/**
+ * Root page that redirects to the appropriate language version
+ * Detects user's preferred language from Accept-Language header
+ */
+export default async function RootPage() {
+  const headersList = await headers();
+  const acceptLanguage = headersList.get('accept-language') || '';
+  
+  // Get configured languages and default language from site config
+  const availableLanguages = siteConfig.i18n.languages;
+  const defaultLanguage = siteConfig.i18n.defaultLanguage;
+  
+  // Parse Accept-Language header to find the best match
+  const preferredLanguage = detectPreferredLanguage(acceptLanguage, availableLanguages);
+  
+  // Redirect to the detected or default language
+  const targetLanguage = preferredLanguage || defaultLanguage;
+  redirect(`/${targetLanguage}/docs`);
+}
+
+/**
+ * Detect preferred language from Accept-Language header
+ * Returns the first available language that matches user preferences
+ */
+function detectPreferredLanguage(
+  acceptLanguage: string,
+  availableLanguages: string[]
+): string | null {
+  if (!acceptLanguage) {
+    return null;
+  }
+  
+  // Parse Accept-Language header
+  // Example: "en-US,en;q=0.9,zh-CN;q=0.8,zh;q=0.7"
+  const languages = acceptLanguage
+    .split(',')
+    .map(lang => {
+      const [code, qValue] = lang.trim().split(';q=');
+      const quality = qValue ? parseFloat(qValue) : 1.0;
+      // Extract primary language code (e.g., "en" from "en-US")
+      const primaryCode = code.split('-')[0].toLowerCase();
+      return { code: primaryCode, quality };
+    })
+    .sort((a, b) => b.quality - a.quality); // Sort by quality (preference)
+  
+  // Find first matching language that is available
+  for (const { code } of languages) {
+    // Check if the language code matches any available language
+    if (availableLanguages.includes(code)) {
+      return code;
+    }
+    // Special case: map 'zh' to 'cn'
+    if (code === 'zh' && availableLanguages.includes('cn')) {
+      return 'cn';
+    }
+  }
+  
+  return null;
+}