Add internationalization (i18n) with next-intl and Italian translations

- Integrated `next-intl` for server-side and client-side i18n support.
- Added English (`en.json`) and Italian (`it.json`) localization files.
- Configured routing with locale-based subdirectories (`/[locale]/path`) using `next-intl`.
- Introduced type-safe i18n utilities and TypeScript definitions for translation keys.
- Updated middleware to handle locale detection and routing.
- Implemented dynamic translation loading to reduce bundle size.
- Enhanced developer experience with auto-complete and compile-time validation for i18n keys.

frontend/src/types/i18n.d.ts

@@ -0,0 +1,25 @@
+// src/types/i18n.d.ts
+/**
+ * TypeScript type definitions for i18n with next-intl.
+ *
+ * This file configures TypeScript autocomplete for translation keys.
+ * By importing the English messages as the reference type, we get:
+ * - Full autocomplete for all translation keys
+ * - Type safety when using t() function
+ * - Compile-time errors for missing or incorrect keys
+ *
+ * Usage:
+ * ```tsx
+ * const t = useTranslations('auth.login');
+ * t('title'); // ✅ Autocomplete shows available keys
+ * t('invalid'); // ❌ TypeScript error
+ * ```
+ */
+
+type Messages = typeof import('../../messages/en.json');
+
+declare global {
+  // Use type safe message keys with `next-intl`
+  // eslint-disable-next-line @typescript-eslint/no-empty-object-type
+  interface IntlMessages extends Messages {}
+}