Add free-text funding source field for "Private funding" Source: TC#030/#033

When "Private funding" is selected under Project Funding/funding sources, no field currently allows specifying the funder's name (e.g., "Michael J. Fox Foundation"). Add a free-text field that appears automatically when "Private funding" is selected.

Start at the Project Funding/funding sources form and trace how the "Private funding" selection is rendered. Add a free-text funder-name field that appears for that selection, then verify the value can be entered and retained when the form is submitted.

Written by the indexing model from the issue text.