Refactor i18n integration and update tests for improved localization

- Updated test components (`PasswordResetConfirmForm`, `PasswordChangeForm`) to use i18n keys directly, ensuring accurate validation messages.
- Refined translations in `it.json` to standardize format and content.
- Replaced text-based labels with localized strings in `PasswordResetRequestForm` and `RegisterForm`.
- Introduced `generateLocalizedMetadata` utility and updated layout metadata generation for locale-aware SEO.
- Enhanced e2e tests with locale-prefixed routes and updated assertions for consistency.
- Added comprehensive i18n documentation (`I18N.md`) for usage, architecture, and testing.

frontend/tests/components/auth/LoginForm.test.tsx

@@ -82,8 +82,7 @@ describe('LoginForm', () => {
     await user.click(submitButton);
 
     await waitFor(() => {
-      expect(screen.getByText(/email is required/i)).toBeInTheDocument();
-      expect(screen.getByText(/password is required/i)).toBeInTheDocument();
+      expect(screen.getAllByText(/this field is required/i).length).toBeGreaterThanOrEqual(2);
     });
   });
 
@@ -100,7 +99,7 @@ describe('LoginForm', () => {
     await user.click(submitButton);
 
     await waitFor(() => {
-      expect(screen.getByText(/password must be at least 8 characters/i)).toBeInTheDocument();
+      expect(screen.getByText(/must be at least 8 characters/i)).toBeInTheDocument();
     });
   });
 
@@ -233,9 +232,7 @@ describe('LoginForm', () => {
       await user.click(submitButton);
 
       await waitFor(() => {
-        expect(
-          screen.getByText('An unexpected error occurred. Please try again.')
-        ).toBeInTheDocument();
+        expect(screen.getByText('unexpectedError')).toBeInTheDocument();
       });
     });