feat(i18n): 결제 금액 다국어 키 추가

lib/l10n/app_localizations.dart

@@ -231,6 +231,8 @@ class AppLocalizations {
   String get saveChanges => _localizedStrings['saveChanges'] ?? 'Save Changes';
   String get monthlyExpense =>
       _localizedStrings['monthlyExpense'] ?? 'Monthly Expense';
+  String get billingAmount =>
+      _localizedStrings['billingAmount'] ?? 'Billing Amount';
   String get websiteUrl => _localizedStrings['websiteUrl'] ?? 'Website URL';
   String get websiteUrlOptional =>
       _localizedStrings['websiteUrlOptional'] ?? 'Website URL (Optional)';