This fixes a bug where tasks which had their done status changed were not moved in the correct bucket. This affected both frontend and api. The move of the task between buckets is now correctly done in the api and frontend - with a bit of duplicated logic between the two. This could be optimized further in the future.
Resolves https://kolaente.dev/vikunja/vikunja/issues/2610
Before this change, the filter button on the top right was positioned using absolute positioning and plenty of tricks, which were brittle and not really maintainable. Now, the buttons are positioned using flexbox, which should make this a lot more maintainable.