fix(api): keep DioExceptionType switch exhaustive across dio versions

CI resolves dependencies fresh (pubspec.lock is gitignored), so the
v1.7.0 build failed when dio 5.10 introduced transformTimeout. A
default clause absorbs future enum additions on either dio version;
unknown keeps identical behavior through it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/core/api/api_interceptors.dart

@@ -163,7 +163,10 @@ class ErrorInterceptor extends Interceptor {
       case DioExceptionType.badCertificate:
         return const NetworkException(message: 'Invalid SSL certificate');
 
-      case DioExceptionType.unknown:
+      // default keeps this exhaustive across dio versions (CI resolves deps
+      // fresh — pubspec.lock is gitignored — so DioExceptionType can gain
+      // cases, e.g. transformTimeout in dio >= 5.8)
+      default:
         return NetworkException(
           message: err.message ?? 'Unknown error',
           cause: err,