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>

public/components/oikos-locale-picker.js

@@ -10,6 +10,7 @@ import { t, setLocale, getLocale, getSupportedLocales } from '/i18n.js';
 const LOCALE_LABELS = {
   de: 'Deutsch',
   en: 'English',
+  it: 'Italiano',
 };
 
 class OikosLocalePicker extends HTMLElement {