Bugfix: Render calculated user start nodes in User Workspace

[madsrasmussen]

Description

This PR implements the new endpoint to fetch the calculated start nodes for a user (umbraco/Umbraco-CMS#16609)

What to test:

• Set up a user and assign user groups that will result in different start nodes.
• Test that the calculated result is shown to the user. Be aware that you might have to restart the server to see the effect (more details in PR for management API (Add endpoint for calculating effective user start nodes Umbraco-CMS#16609)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (minor updates related to the tooling or maintenance of the repository, does not impact compiled assets)

[iOvergaard]

Looks and tests good. I tried several scenarios with varying degrees of start nodes. The only thing I could find is that you have to click out of the user and back into them when you change their user groups to get the UI to call the "calculate-start-nodes" endpoint again. Perhaps we could listen on that when user groups change. Approved nonetheless!