fix: risolvi 6 vulnerabilità npm aggiornando vite e vitest

- Upgrade vite 5 → 6 (fix GHSA-67mh-4wv8-2f99: esbuild CORS dev server)
- Upgrade vitest 2 → 3 e @vitest/coverage-v8 2 → 3
- Rinomina vite.config.js → vite.config.mjs (richiesto da vite 6 ESM)
- Aggiorna requisito Node.js in README: 18.x → >= 20.x LTS

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -16,12 +16,12 @@
   },
   "devDependencies": {
     "@playwright/test": "^1.58.2",
-    "@vitejs/plugin-vue": "^5.0.0",
-    "@vitest/coverage-v8": "^2.1.9",
+    "@vitejs/plugin-vue": "^5.2.0",
+    "@vitest/coverage-v8": "^3.0.0",
     "@vue/test-utils": "^2.4.6",
     "happy-dom": "^20.8.9",
     "jsdom": "^29.0.1",
-    "vite": "^5.0.0",
-    "vitest": "^2.1.9"
+    "vite": "^6.0.0",
+    "vitest": "^3.0.0"
   }
 }