leaderboard: add Tasks tab to browse benchmark fixtures

stage shards via bucket API instead of volume mount

stage shard artifacts through HF bucket

step 10 / jobs-migration: requirements.txt -> dispatch eval to HF Jobs

submit+app: gate Submit on HF OAuth; populate hf_username from profile

app+requirements: pin Gradio 5 + gradio_leaderboard, auto-refresh now works

leaderboard: fetch results.jsonl via direct cache-busted HTTPS

step 3: gradio leaderboard scaffold