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.

package.json

@@ -14,6 +14,7 @@
     "@nuxt/eslint": "1.10.0",
     "@nuxt/ui": "4.1.0",
     "@nuxtjs/i18n": "10.2.0",
+    "@nuxtjs/seo": "3.2.2",
     "better-sqlite3": "^12.4.1",
     "eslint": "^9.39.0",
     "nuxt": "^4.2.0",