feat(i18n): add Italian localization

Add complete Italian translation (497 keys) based on PR #7 by
@albanobattistella. Fixed filename from "it. json" to "it.json" and
registered Italian in SUPPORTED_LOCALES and the locale picker component.

Co-Authored-By: albanobattistella <34811668+albanobattistella@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "oikos",
-  "version": "0.5.7",
+  "version": "0.5.8",
   "description": "Selbstgehosteter Familienplaner — Kalender, Aufgaben, Einkauf, Essensplan, Budget und mehr. Privat, offen, ohne Abo.",
   "main": "server/index.js",
   "engines": {