Allow actor selection to be passed as a URL parameter, enabling direct redirection into the form via a link

### Description

Mattilsynet requests a button link on the receipt page of the application that enables users to return directly to the service and submit a new form. Currently, users can only be redirected to the service start page, where they must manually reselect the actor.

For inbox scenarios, this functionality is already supported, with URN --> https://af.tt02.altinn.no/?party={partyURN}, which takes the user directly to the selected actor.

While one actor may be submitting 20–30 applications, this feature would have been a improvement in terms of efficiency and user experience.

See the full conversation here: https://digdir-samarbeid.slack.com/archives/C02EJ9HKQA3/p1778663861135279?thread_ts=1776755556.945129&cid=C02EJ9HKQA3

Related app: https://altinn.studio/repos/mat/akvakultur-driftsplan

### Additional Information

_No response_

Start in the receipt-page return-link flow in Altinn/app-frontend-react and compare it with the existing inbox handling of the `?party={partyURN}` URL parameter. The change is complete when a link can pass the actor selection and open a new form directly for that actor, avoiding the service start page and manual reselection.

Written by the indexing model from the issue text.