Technical: Upgrade to NextJS 15

[n0rdlicht]

To make solving #91 easier we should upgrade to NextJS 15. From the upgrade notes:

React 19

useFormState has been replaced by useActionState. The useFormState hook is still available in React 19, but it is deprecated and will be removed in a future release. useActionState is recommended and includes additional properties like reading the pending state directly. Learn more.
useFormStatus now includes additional keys like data, method, and action. If you are not using React 19, only the pending key is available. Learn more.

Read more in the React 19 upgrade guide.

[n0rdlicht]

Currently blocked to peer dependencies rc status of react 19 and nextjs 15