feat(i18n): add locale management and configuration

src/config/i18n.ts

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+// Supported locales
+export const locales = ["en", "zh"] as const;
+export type Locale = (typeof locales)[number];
+
+// Default locale
+export const defaultLocale: Locale = "en";
+
+// Cookie key for storing selected locale
+export const LOCALE_COOKIE_NAME = "LOCALE";

src/i18n/locale.ts

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+"use server";
+
+import {
+  defaultLocale,
+  Locale,
+  LOCALE_COOKIE_NAME,
+  locales
+} from "@/config/i18n";
+import { cookies, headers } from "next/headers";
+
+export async function getUserLocale() {
+  const cookieLocale = (await cookies()).get(LOCALE_COOKIE_NAME)?.value;
+
+  if (cookieLocale && locales.includes(cookieLocale as Locale)) {
+    return cookieLocale as Locale;
+  }
+
+  const acceptLanguage = (await headers()).get("accept-language") || "";
+  const firstLang = acceptLanguage.split(",")[0]?.trim().toLowerCase();
+  const langPrefix = firstLang?.slice(0, 2);
+
+  if (locales.includes(langPrefix as Locale)) {
+    return langPrefix as Locale;
+  }
+
+  return defaultLocale;
+}
+
+export async function setUserLocale(locale: Locale) {
+  (await cookies()).set(LOCALE_COOKIE_NAME, locale);
+}

src/i18n/request.ts

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+import { getUserLocale } from '@/i18n/locale';
+import { getRequestConfig } from 'next-intl/server';
+
+export default getRequestConfig(async () => {
+  const locale = await getUserLocale();
+
+  return {
+    locale,
+    messages: (await import(`../../messages/${locale}.json`)).default
+  };
+});