[Tentative] Playground Builder: CONFIG editing

[rseldner]

Expose CONFIG fields in the Builder tab as form fields, similar to how TREE nodes are editable today.

first attempt:

• title, subtitle, icon (text inputs) done in bfb2404
• palette color values (color pickers or text inputs)
• nodeTypes — at minimum, the type dropdown in node forms should reflect CONFIG.nodeTypes keys dynamically rather than a hardcoded list

title, subtitle, icon would be quick wins. full pallette is more involved.