leaderboard+app: combined CSV download with validation_status discriminator

app: render reports inline via iframe srcdoc (private-Space safe)

app+leaderboard: link submission_name to a Space-side report proxy

app+leaderboard: detail panel polish (rename link columns, fix broken report links)

leaderboard: markdown link columns for agent_url, submission, report

submit: default validation + hf_username fields on the pending row

app: split Leaderboard into Validated + Unvalidated tables

leaderboard: add load_leaderboard_split() for two-tier viewer

tests: pytest + playwright headless smoke harness