Evaluation UI: Show cost #39

Is your feature request related to a problem?

After running an evaluation, the cost information is available in Langfuse but not visible in our UI. This is valuable information for
users to understand the expense of their evaluation runs.

Describe the solution you'd like

Display evaluation cost in the UI. Two approaches:

1. Fetch from Langfuse - Retrieve cost data from Langfuse API when fetching results
2. Calculate locally - Compute cost ourselves once results are returned, based on token usage and model pricing

Additional context

Cost visibility helps users make informed decisions about running large evaluations and tracking expenses.