Credit Cost Estimator in Workflow Builder

As a user adds nodes to a workflow, a small display dynamically updates to show the minimum fixed credit cost for one run of that workflow (e.g., "5 steps * 3 credits/step = 15 credits").

It should also clearly flag if built-in LLM nodes are used, as these incur variable costs. Maybe including tokens estimator as well but this might be secondary for now.

Helps users understand the basic cost implications of their workflow design before running it and consuming credits.