Implement automatic language detection and redirection

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

Author: Copilot

app/page.tsx

@@ -1,5 +1,46 @@
 import { redirect } from 'next/navigation';
+import { headers } from 'next/headers';
+import { i18n } from '@/lib/i18n';
 
-export default function HomePage() {
-  redirect('/en/docs');
+export default async function HomePage() {
+  const headersList = await headers();
+  const acceptLanguage = headersList.get('accept-language') || '';
+  
+  // Parse the Accept-Language header and find the best match
+  const preferredLang = getPreferredLanguage(acceptLanguage, i18n.languages);
+  
+  redirect(`/${preferredLang}/docs`);
+}
+
+function getPreferredLanguage(acceptLanguage: string, availableLanguages: readonly string[]): string {
+  // Parse Accept-Language header (e.g., "zh-CN,zh;q=0.9,en;q=0.8")
+  const languages = acceptLanguage
+    .split(',')
+    .map(lang => {
+      const parts = lang.trim().split(';');
+      const code = parts[0];
+      const quality = parts[1] ? parseFloat(parts[1].split('=')[1]) : 1.0;
+      return { code, quality };
+    })
+    .sort((a, b) => b.quality - a.quality);
+
+  // Try to find the best match (exact or partial) respecting quality order
+  for (const lang of languages) {
+    // Try exact match first
+    if (availableLanguages.includes(lang.code)) {
+      return lang.code;
+    }
+    
+    // Try partial match (e.g., "zh" matches "zh-CN")
+    const langPrefix = lang.code.split('-')[0].toLowerCase();
+    const match = availableLanguages.find(available => 
+      available.toLowerCase().startsWith(langPrefix)
+    );
+    if (match) {
+      return match;
+    }
+  }
+
+  // Default to the configured default language
+  return i18n.defaultLanguage;
 }