This commit integrates the `@nuxtjs/seo` module to establish a baseline for search engine optimization.
Key changes include:
- Installation and configuration of the `@nuxtjs/seo` module.
- Definition of default meta tags for title, description, keywords, Open Graph, and Twitter cards.
- Addition of a default Open Graph image.
- Renaming of `robots.txt` to `_robots.txt` to align with the module's conventions for static file handling.
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.
