Deleted

Just a FYI.
It's not correct all the time, sadly.. For example, app/301540 has steam trading cards. Also this app/467000 and there's more like these.
Only way to know I guess would be store page where there's badge progress. (or using something like steamcardexchange.net)

Since you already made the color picking option, is there any reason not to include the ability to pick color for cards / no cards? Red and green can look pretty similar for some people.

Some games for example https://www.steamgifts.com/game/dldgj/moonstone-island say that don't have trading cards.

I changed the detection to use SteamCardExchange and search the tittle of the HTML (the website doesn't have an API)

  // Returns: true / false / null (null = unknown/error)
  async function fetchHasCards(appid){
    return await new Promise(resolve=>{
      GM_xmlhttpRequest({
        method:'GET',
        url:`https://www.steamcardexchange.net/index.php?gamepage-appid-${appid}/`,
        timeout:15000,
        onload:(res)=>{
          try{
            resolve(res.responseText.search("<title>Showcase :: Game not found!</title>") == -1 ? true : false);
          }catch{ resolve(null); }
        },
        onerror:()=>resolve(null),
        ontimeout:()=>resolve(null),
      });
    });
  }

Also changed this two lines in Cache to use the value from LS.cachePrefix instead of the hard coded 'sgc_app_'

  const keyFor = (appid) => LS.cachePrefix + appid;
  function clearCache(){ Object.keys(localStorage).forEach(k => { if (k.startsWith(LS.cachePrefix)) localStorage.removeItem(k) }); }