feat(hf-space): leaderboard app for the OCB dataset

[Flotapponnier]

Summary

Gradio app + GH Actions deploy for a companion HF Space sitting on top of
the OpenChainBench/benchmarks dataset. Goal is more eyeballs on the
dataset (HF trending, backlinks, search) without any extra moving piece
on our side.

The app reads four parquet partitions live via pl.scan_parquet("hf://...")
each time the Space cold-starts. No local cache, no auth, no bespoke
state. Four tabs:

• Today's leaderboard, sortable + filterable by category, title rendered
  as a link to the bench page on openchainbench.com.
• Per-chain leaders, filterable by bench and chain.
• Provider rankings per bench slug with p50 / p90 / p99 / success rate.
• About page with dataset / repo / site links, BibTeX, CC-BY-4.0.

A workflow on main pushes the folder to
huggingface.co/spaces/OpenChainBench/leaderboard when anything under
scripts/hf_space/ changes. Reuses the existing HF_TOKEN secret.

Test plan

• pip install -r requirements.txt && python app.py boots, serves
  HTTP 200 on 127.0.0.1:7860.
• /config exposes the 3 dataframes prefilled (26 headlines, 39
  chain leader rows, 5 providers on the default slug).
• Filter callbacks via /call/view_headlines and
  /call/view_providers return the expected filtered rows.
• HF Space deploys cleanly on first run (will verify on merge).