feat(seo): integrate @nuxtjs/seo for basic SEO

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.

docs/design-doc-v1.md

@@ -3,6 +3,8 @@
 ### 🔹 品牌主轴：
 
 > 「一家融合创意与技术的定制开发工作室，专注于网站、软件、游戏与交互体验的全栈实现。」
+> A creative technology studio offering **custom development services**, specializing in full-stack solutions for websites, software, games, and interactive experiences.
+
 
 ### 🔹 品牌关键词：