Integrates the Evalgen Backend with Report Card and Multi-Eval Node.

[shreyashankar]

This PR integrates the backend (executor.ts) with the Report card, allowing users to select & inspect candidate implementations. The selected evals get propagated to the multi-eval node.

Things I haven't figured out:

• Not quite sure what the ring progress indicators are supposed to do.
• Saving grades in session storage. I think we can probably integrate something into the front end, so we have it for UIST, and then figure out how to save grades/persist beyond a single grading session.