Fix WebUI not handling DOWNSTREAM_SERVICE_ERROR from Hive Gateway for GraphQL errors

## Problem

When GraphQL errors pass through the Hive Gateway, the gateway rewrites extensions.code to DOWNSTREAM_SERVICE_ERROR and adds serviceName field. The WebUI does not recognize this error format, causing the page to break.

Direct Manager responses (without gateway) use the original Backend.AI error code in extensions.code (e.g. backendai_read_not-found), which the WebUI handles correctly.

## Error Formats

Format that does NOT work (via Gateway):

```json
{"extensions": {"code": "DOWNSTREAM_SERVICE_ERROR", "serviceName": "graphene"}}
```

Format that works (direct from Manager):

```json
{"extensions": {"service": "graphene"}}
```

## Root Cause

Hive Gateway (gateway.config.ts, maskedErrors: false) passes through error messages but transforms the extensions structure. The WebUI error handler branches on extensions.code and does not recognize DOWNSTREAM_SERVICE_ERROR.

## Fix Direction

1. WebUI: Add handling for DOWNSTREAM_SERVICE_ERROR code, fall back to message-based error handling
1. Gateway: Add custom plugin to preserve original extensions.code from subgraph errors

JIRA Issue: BA-4863