LiveData path validation failure is not surfaced to the user

The form - to create a new path or edit an existing one - is not properly backed by a changeset. I think this is an artifact from a previous data model prototype, and there is currently no reason for it to not use `LiveDataAssign.changeset/2`

The effect of this issue is that attempts to create or update Live Data with an invalid path will fail silently, without showing any error message to the user.

NOTE that this will require upstream changes from beacon, as the full changeset is not publicly exposed right now, only the path changeset.

Start with the form paths for creating and editing Live Data, then inspect how LiveDataAssign.changeset/2 is used. Coordinate the required upstream Beacon change to expose the full changeset; this is done when invalid paths produce visible validation errors instead of failing silently.

Written by the indexing model from the issue text.