docs: annotate confidence contract drift and X-Real-IP trust assumption (#52)

feat(frontend): detect HF Space cold start and auto-retry once awake

feat(frontend): el proxy /api/query reenvía X-Proxy-Secret y X-Real-IP al backend

fix(frontend): envia card_mentions al backend para lookup directo de cartas

feat(hardening): frontend error states -- typed errors + ErrorDisplay component

feat: scaffold Next.js frontend with lib, store, and API proxy route