Agent i18n:
- All command responses bilingual (help, status, positions, balance, trades, analysis, settings, errors)
- Language auto-detected from web UI lang toggle
- Stored per-user in SQLite preferences
- AI chat system prompt switches to Chinese/English based on user lang
- Trade confirmation prompts bilingual
Web UI:
- Sends lang parameter with every API request
- Agent extracts [lang:xx] prefix and persists preference
i18n message catalog: nofxi/internal/agent/i18n.go
