Remove i18n, keep only Italian and rely on browser translation

The interface was translated in en/it/es/fr via @nuxtjs/i18n. Drop the
module, the locale files, and the language switcher: the UI is now
static Italian text, with translation for foreign visitors left to the
Google Translate browser extension instead of the app.

frontend/package.json

@@ -11,7 +11,6 @@
     "typecheck": "nuxt typecheck"
   },
   "dependencies": {
-    "@nuxtjs/i18n": "^10.6.0",
     "marked": "^18.0.11",
     "nuxt": "^4.5.2",
     "simple-icons": "^16.28.0",