Refactored Frontend and split js into separat file

[Arne1303]

The frontend got quite crowded, so I refactored it to be more compact, this also removes the padding on top in favor of a flex box which centers everything vertically. In addition to that, I also extracted the JS code into a separate file app.js to make it easier to maintain (The App still runs flawlessly with python -m http.server 3000). I think this step is necessary in the long run, it does however conflict with all other PRs which also change something on the Frontend (Mainly https://github.com/FujiwaraChoki/MoneyPrinter/pull/130 and https://github.com/FujiwaraChoki/MoneyPrinter/pull/116)

[Arne1303]

@FujiwaraChoki Im ready to solve the merge conflicts if you want to merge this at any point, just write a comment that all other work on the front end is done so that I only have to do it once

[FujiwaraChoki]

Actually, thank you for the idea, I'll just do it myself right now, sorry it took so long!