Add Docker Compose setup and bridge-network device discovery

[tuyndoan]

Summary

• Complete the multi-stage Dockerfile (copy React build, expose port 7020, add startup command).
• Add docker-compose.yml for local deployment with persistent ./data volume.
• Add DISCOVERY_HOSTS env var for direct Broadlink probing when UDP broadcast discovery fails on Docker bridge networks.
• Add missing cryptography dependency and sync frontend package-lock.json for reproducible Docker builds.

Test plan

• docker compose up --build -d builds and starts successfully
• App reachable at http://localhost:7020
• /api/version returns 200
• With DISCOVERY_HOSTS set to a known device IP, /autodiscover?freshscan=1 returns the device
• IR learn/send still works against a discovered device