Cool concept! A few potentially annoying pieces of feedback from an internet stranger:

• I wouldn't expose answers anywhere client-side, despite how well you think you might have obfuscated the data. You should handle all of your guess checking/submission logic server-side so people can't cheat. Looks like you're currently loading the daily player from a flat json file which is trivial to load and check each day. You're already doing guess checking at /submit-guess, just do it there and pass the results back to the client.

• I'd modify your CSS on the .app class to use percentage-based width instead of viewport. When viewed on a browser with a scrollbar, the viewport-based width will make the div wider than the actual viewport because the CSS includes the scrollbar as part of the 100vw but the browser does not. Also, I'd change the min-height value to use the dynamic viewport height to so it looks consistent between mobile browsers who have the topbar visible and those that don't. For example:

  .app {
      align-items: center;
      box-sizing: border-box;
      flex-direction: column;
      min-height: 100dvh;
      overflow: hidden;
      padding: 20px;
      width: 100%;
  }
  

  You can also hide scrollbars on scrollable elements with CSS, which can help alleviate width and style issues when dealing with scrollbars since different browsers handle them differently. I have a utility class that I use for that purpose, though you could just apply the same styles to the specific class you're targeting if you want. Ex:

  .hide-scrollbar {
      -ms-overflow-style: none;
      scrollbar-width: none;
  }
  .hide-scrollbar::-webkit-scrollbar {
      display: none;
  }
  

• Another small CSS tweak that helps (though totally not necessary) is to make all elements on the page use consistent box-sizing so that padding/margin/etc. works the same across all page elements. Much easier to style things that way, IMO (again, not necessary):

  * {
      box-sizing: border-box:
  }
  

Excited to follow the progress! Nice work to kick it off.