feat: Add dashboard controls for deleting local runs

[GO1984]

Summary

Adds dashboard controls for deleting persisted local benchmark runs.

Changes

• Add deleteRun(runId) API helper for DELETE /api/benchmark/runs/{run_id}.
• Add a Delete button on the run detail page.
• Add a Delete action in the local leaderboard table.
• Ask for confirmation before deleting.
• Show API errors inline if deletion fails.
• Navigate back to the leaderboard after deleting from the detail page.
• Refresh the leaderboard after deleting from the table.

Why

BenchLoop already lists and opens local benchmark runs, including partial and failed runs, but there is no UI action to delete a single stored run. Users currently need to manually remove folders from ~/.bench-loop/runs/.

This makes cleanup easier for accidental, partial, failed, or obsolete local runs.

Backend dependency

This UI expects the local dashboard API to provide:

DELETE /api/benchmark/runs/{run_id}

Matching backend PR:

outsourc-e/bench-loop#16

Validation

npm run build
success