Mirror dungeon definitions in web lobby

[jackandcarter]

Summary

• add shared difficulty definitions and floor room rules derived from the database seed data, and expose them via /api/difficulties
• update dungeon generation to honor database-driven difficulty settings, room kinds, and boss placement while serializing richer floor metadata
• refresh the lobby UI to load dynamic difficulties, display dungeon legends, and show new floor/difficulty details for sessions

Testing

• npm run lint (fails: eslint config missing in repo)
• npm run build (fails: missing TypeScript/node type dependencies in the environment)

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Codex Task