Party selection: search for subunits

### Description

Currently it is not possible to search for subunits. If an org has many sub units it can be hard to find the one you're after.

When "Show sub units" is checked, the top-level orgs should also be shown if any of its sub units match the search string. The sub units then also need to be filtered.

### Additional Information

_No response_

Start by locating the party selection component and its existing search and "Show sub units" filtering behavior. Verify how organizations and subunits are represented, then confirm the result shows a top-level organization when a subunit matches and filters the displayed subunits to matching entries.

Written by the indexing model from the issue text.