FE: Delete page groups

[Jondyr]

Backend endpoint implemented in #15030
When deleting the page groups, delete everything inside the group. Let the user know that there is content inside before making the deletion.

Decide what the text content should display in the native confirm popup, i.e. "Er du sikker på at du vil slette sidegruppen, da vil alt innhold i den også slettes". Maybe we can reuse the text from native confirm when deleting a page.

Why

As a user, I sometimes need to delete page groups — for example, if they're no longer relevant or were added by mistake. When doing so, I want to be clearly informed that all pages and components inside the group will also be deleted. This helps prevent accidental data loss and gives me confidence in what the delete action will do.

Design

[wrt95]

When deleting the page groups, delete everything inside the group. Let the user know that there is content inside before making the deletion