fix: handle json parse exceptions on 404/500 signup responses gracefully

userfront/lib/core/services/auth_proxy_service.dart

@@ -990,8 +990,17 @@ class AuthProxyService {
     );
 
     if (response.statusCode != 200) {
-      final error = jsonDecode(response.body)['error'] ?? 'Signup failed';
+      String errorMessage = 'Signup failed';
-      throw Exception(error);
+      try {
+        final decoded = jsonDecode(response.body);
+        if (decoded is Map<String, dynamic> && decoded.containsKey('error')) {
+          errorMessage = decoded['error'];
+        }
+      } catch (e) {
+        // Fallback if the body isn't valid JSON (e.g., an HTML error page)
+        errorMessage = 'Server error (${response.statusCode}): ${response.body.isNotEmpty ? response.body.substring(0, response.body.length > 100 ? 100 : response.body.length) : "Unknown error"}';
+      }
+      throw Exception(errorMessage);
     }
   }
 }