Unify leaderboard selectors into a single controls bar

[radinhamidi]

Summary

• Replace the two stacked cards (selector card + filter card) on the leaderboard home page with a single controls bar: Benchmark / Dataset / Retriever on row 1, LLM / Method / Search on row 2, separated by vertical dividers.
• Move LLM and Method from chip groups to <select> dropdowns (options populated from the real models/methods data).
• Point the leaderboard footer's Paper link at the reproducibility paper (arXiv 2604.27421) via a new paperHref prop on the shared Footer. The marketing site keeps the toolkit paper (2511.15996).

UI-only: no data, results, scores, schema, or library code touched. The four per-X pages are unchanged.

Test plan

• Default load: MS MARCO + DL 2019 + Retriever All
• LLM / Method dropdowns filter the table
• Benchmark pills repopulate the Dataset dropdown; Retriever pills toggle the retriever column
• Search + sort headers still work
• Light and dark mode
• Leaderboard footer Paper link → arxiv.org/abs/2604.27421; marketing footer still → 2511.15996

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