Technical debt

During the first 6 months of the project, it accumulated more and more technical debt. At some point, it needed to be corrected, and this branch aims to do that. It updates almost every file, changes the way requests are made to the API, updates sveltekit and all its dependencies to their latest versions, fixes many latent bugs, addresses security issues, and updates some designs.

However, since it has changed many things, although I have tested most of the features, I have yet to confirm that some of them work again. Also, I will wait for #69 (closed), #133 (closed) and #135 (closed) to be resolved and handle merge conflicts.

Closes #36 (closed).