Use Svelte runes instead of Svelte stores and reactive variables

Svelte is currently designing the `runes` system to replace the current paradigm of using stores and reactive variables. It is probably released and stable in one year.
I would recommend the next group to look into runes, and port the existing code to use them. Runes should be better™ overall, and porting it now will be easier compared to doing it when the entire app is finished and relies heavily on stores.

More reading: https://svelte.dev/blog/runes