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>

vite.config.mjs

@@ -0,0 +1,6 @@
+import { defineConfig } from 'vite'
+import vue from '@vitejs/plugin-vue'
+
+export default defineConfig({
+  plugins: [vue()]
+})