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.

package.json

@@ -13,6 +13,7 @@
     "@nuxt/content": "3.8.0",
     "@nuxt/eslint": "1.10.0",
     "@nuxt/ui": "4.1.0",
+    "@nuxtjs/i18n": "10.2.0",
     "better-sqlite3": "^12.4.1",
     "eslint": "^9.39.0",
     "nuxt": "^4.2.0",