Create CRUD endpoints for pages/groups

[Jondyr]

Description

Instead of sending multiple requests and also sending the whole (and potentially outdated) Settings.json file upon doing changes to pages, we should have backend endpoints for just those specific actions.

Additional Information

This lets the backend do the heavy lifting and avoids issues, for example from multiple requests being sent quickly (like this issue https://digdir.slack.com/archives/C077KTTCBSS/p1738755536603919) or one of the requests failing, leaving the app in a broken state.

Testing

Deploy the top branch of the PR stack (currently #14717) and test:

• Page modifying operations on the frontend
• Multiple requests to page related endpoints on a slow internet connection does not result in invalid states or exceptions
• Page re-ordering
• Layout editing works after modifying pages (order, name or addition/deletion)

Tasks

example of needed endpoint routes:
POST /layout/pages/create
DELETE /layout/{layoutname}/page/{pageid}
PUT /layout/{layoutname}/page/{pageId}
GET not needed? how do we avoid stale caches being mutated without this, optimistic cache updates?

Also implement these as tanstack mutations, and change the frontend logic to use these (and invalidate layout caches?)

Acceptance Criterias

No response