Send 504 Gateway Timeout error when response timeout occurs

- Provides proper HTTP response instead of closing connection
- Prevents browser 'message port closed' errors

wssshd2/web_proxy.c

@@ -649,6 +649,7 @@ static int handle_proxy_request(int client_fd, wssshd_state_t *state, const wsss
             if (activity == 0) {
                 // Timeout
                 printf("[WEB-PROXY] Timeout waiting for response from wsssht\n");
+                send_http_error(client_fd, 504, "Gateway Timeout");
                 break;
             }