Add locale switcher component and integrate internationalization improvements

- Introduced `LocaleSwitcher` component for language selection with support for locale-aware dropdown and ARIA accessibility.
- Updated layouts (`Header`, `Breadcrumbs`, `Home`) to include the new locale switcher.
- Expanded localization files (`en.json`, `it.json`) with new keys for language switching.
- Adjusted i18n configuration to enhance routing and message imports.
- Updated Jest module mappings to mock new i18n components and utilities.

frontend/src/components/admin/Breadcrumbs.tsx

@@ -25,6 +25,7 @@ export function Breadcrumbs() {
   const pathname = usePathname();
 
   // Generate breadcrumb items from pathname
+  // Note: usePathname() from next-intl returns path WITHOUT locale prefix
   const generateBreadcrumbs = (): BreadcrumbItem[] => {
     const segments = pathname.split('/').filter(Boolean);
     const breadcrumbs: BreadcrumbItem[] = [];