FormControls: fix error for preview prop type

[distantnative]

Todo

• https://github.com/getkirby/kirby/blob/v5/changes/develop/panel/src/components/Views/Pages/SiteView.vue#L23 references a site preview permission which is also mentioned in the docs. However, this neither does exist in SiteBlueprint nor has it been used in the past to control the display of the preview button on the site view https://github.com/getkirby/kirby/blob/main/panel/src/components/Views/Pages/SiteView.vue#L17-L25
  • Are we introducing this new permission and need to properly implement it in all the places (blueprint, area buttons)?
  • Or do we need to remove this here in site view and from the docs?

Summary of changes

• k-form-controls accepts a boolean (false) for preview prop