feat(i18n): add internationalization support

This commit introduces internationalization (i18n) support by integrating the `@nuxtjs/i18n` module. It configures English (en) and
Simplified Chinese (zh-CN) locales, adds a language selector to the header, and internationalizes the index page content using
translation files.

nuxt.config.ts

@@ -2,6 +2,23 @@
 export default defineNuxtConfig({
   compatibilityDate: "2025-07-15",
   devtools: { enabled: true },
-  modules: ["@nuxt/content", "@nuxt/ui", "@nuxt/eslint"],
+  modules: ["@nuxt/content", "@nuxt/ui", "@nuxt/eslint", "@nuxtjs/i18n"],
   css: ["@/assets/css/main.css"],
+  i18n: {
+    defaultLocale: "en",
+    locales: [
+      {
+        code: "en",
+        iso: "en-US",
+        name: "English",
+        files: ["en-US/index.json"],
+      },
+      {
+        code: "zh-CN",
+        iso: "zh-CN",
+        name: "简体中文",
+        files: ["zh-CN/index.json"],
+      },
+    ],
+  },
 });