Use selection instead of plain text boxes when constraints applies

We currently have constraints on the label that can be assigned to a link. Can we render attribute selection from the constraints instead of making users to type in a text box.

Start by locating the link-label constraint handling and the UI that currently renders plain text boxes. Trace how constraints reach that view, then verify that constrained labels render as selections while unconstrained labels retain their current input behavior.

Written by the indexing model from the issue text.