feat: support HTTP_PROXY environment variable for default HTTP client

- Add Proxy: http.ProxyFromEnvironment to default transport
- Allow users to set global proxy via Docker environment variables
- Per-channel proxy settings still override global proxy
- Fully backward compatible

service/http_client.go

@@ -38,6 +38,7 @@ func InitHttpClient() {
 		MaxIdleConns:        common.RelayMaxIdleConns,
 		MaxIdleConnsPerHost: common.RelayMaxIdleConnsPerHost,
 		ForceAttemptHTTP2:   true,
+		Proxy:               http.ProxyFromEnvironment, // Support HTTP_PROXY, HTTPS_PROXY, NO_PROXY env vars
 	}
 
 	if common.RelayTimeout == 0 {