Add web/public/data/predictions/ directory with demo JSON files:
- leaderboard.json: demo predictor leaderboard entries
- open.json: demo open matches for predictions
These placeholder files follow the same pattern as other demo data
(e.g., bots/index.json, leaderboard.json) for local development
and consistency. The index builder generates the real versions at
runtime via generatePredictionsIndex() and generatePredictionsOpen().
Closes: bf-4w95
