// best guess for our main URL (with trailing /)
// should improve (return actual server name!)
static S mainURL() {
  O bot = getBot("#1002596"); // System URL Bot
  if (bot != null) pcall {
    S url = (S) call(bot, "getBaseURL");
    if (nempty(url))
      ret addSlash(url);
  }
  int port = webServerPort();
  ret "http://" + myBestIP() + (port == 80 ? "" : ":" + port) + "/";
}