feat(authentication): use localized strings for demo hints

This change updates the authentication UI pages to use the newly added localization keys (`demoEmailHint` and `demoCodeHint`) instead of hardcoded strings.

This ensures that the demo hints are properly translated for both English and Arabic, improving the internationalization of the application.

Author: fulleni

lib/authentication/view/email_code_verification_page.dart

@@ -82,7 +82,7 @@ class EmailCodeVerificationPage extends StatelessWidget {
                               children: [
                                 const SizedBox(height: AppSpacing.md),
                                 Text(
-                                  'For demo, use code: 123456',
+                                  l10n.demoCodeHint('123456'),
                                   style: textTheme.bodyMedium?.copyWith(
                                     color: colorScheme.secondary,
                                     fontWeight: FontWeight.bold,

lib/authentication/view/request_code_page.dart

@@ -139,7 +139,7 @@ class _RequestCodeView extends StatelessWidget {
                             return Padding(
                               padding: const EdgeInsets.only(top: AppSpacing.lg),
                               child: Text(
-                                'For demo, use email: [email protected]',
+                                l10n.demoEmailHint('[email protected]'),
                                 style: textTheme.bodyMedium?.copyWith(
                                   color: colorScheme.secondary,
                                   fontWeight: FontWeight.bold,