Add Leaderboard CSV / JSON Export button in Web Dashboard

[Kaap10]

Description

Researchers, contributors, and evaluators using AgentBench need to export leaderboard rankings, model performance metrics, and cost ratios into .csv and .json formats for use in reports, presentations, and publications.

Proposed Solution

Add an "Export Leaderboard" button to the top-right of frontend/src/pages/Leaderboard.tsx:

• Export as CSV: Generates a clean comma-separated values file containing Rank, Agent Name, Model, Provider, Total Runs, Success Rate (%), Avg Score.
• Export as JSON: Generates a structured JSON dump of the leaderboard data.

Relevant Files

• frontend/src/pages/Leaderboard.tsx
• frontend/src/lib/api.ts

Acceptance Criteria

• Clean button placed on the Leaderboard header.
• Clicking triggers client-side download without page reload.
• TypeScript build passes with zero errors (npm run build).