feat(nofxi): full i18n bilingual support (zh/en)

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

nofxi/internal/agent/i18n.go

@@ -0,0 +1,204 @@
+package agent
+
+// i18n message templates
+var messages = map[string]map[string]string{
+	"help": {
+		"zh": `🤖 *NOFXi — 你的 AI 交易 Agent*
+
+*交易：*
+/buy BTC 0.01 — 做多（市价单）
+/sell BTC 0.01 — 做空
+/close BTC — 平仓
+/positions — 查看持仓
+/balance — 查看余额
+/pnl — 盈亏记录
+
+*分析：*
+/analyze BTC — AI 市场分析
+/watch BTC — 监控价格
+/alert BTC above 100000 — 价格提醒
+/price BTC — 实时价格
+
+*策略：*
+/strategy start BTC 1h — 启动 AI 自动策略
+/strategy list — 查看运行中的策略
+/strategy stop <id> — 停止策略
+
+*系统：*
+/status — Agent 状态
+/help — 帮助菜单
+
+直接跟我说话就行，中英文都可以 💬`,
+
+		"en": `🤖 *NOFXi — Your AI Trading Agent*
+
+*Trading:*
+/buy BTC 0.01 — Open long (market order)
+/sell BTC 0.01 — Open short
+/close BTC — Close position
+/positions — View open positions
+/balance — Check balance
+/pnl — P/L history
+
+*Analysis:*
+/analyze BTC — AI market analysis
+/watch BTC — Monitor price
+/alert BTC above 100000 — Price alert
+/price BTC — Real-time price
+
+*Strategy:*
+/strategy start BTC 1h — Start AI auto strategy
+/strategy list — View active strategies
+/strategy stop <id> — Stop strategy
+
+*System:*
+/status — Agent status
+/help — This menu
+
+Just talk to me in any language 💬`,
+	},
+
+	"no_trades": {
+		"zh": "📭 暂无交易记录。试试 `/buy BTC 0.01` 或 `/analyze BTC` 开始吧。",
+		"en": "📭 No trades yet. Start with `/buy BTC 0.01` or `/analyze BTC`.",
+	},
+	"no_positions": {
+		"zh": "📭 当前没有持仓。",
+		"en": "📭 No open positions.",
+	},
+	"recent_trades": {
+		"zh": "📋 *最近交易*\n\n",
+		"en": "📋 *Recent Trades*\n\n",
+	},
+	"total_pnl": {
+		"zh": "\n💰 总盈亏: $%.2f",
+		"en": "\n💰 Total P/L: $%.2f",
+	},
+	"open_positions": {
+		"zh": "📊 *当前持仓*\n\n",
+		"en": "📊 *Open Positions*\n\n",
+	},
+	"total_unrealized": {
+		"zh": "💰 *未实现总盈亏: $%.2f*",
+		"en": "💰 *Total Unrealized P/L: $%.2f*",
+	},
+	"account_balance": {
+		"zh": "💰 *账户余额*\n\n",
+		"en": "💰 *Account Balance*\n\n",
+	},
+	"balance_total": {
+		"zh": "   总额: $%.2f\n",
+		"en": "   Total: $%.2f\n",
+	},
+	"balance_available": {
+		"zh": "   可用: $%.2f\n",
+		"en": "   Available: $%.2f\n",
+	},
+	"balance_in_position": {
+		"zh": "   持仓占用: $%.2f\n\n",
+		"en": "   In Position: $%.2f\n\n",
+	},
+	"no_exchange": {
+		"zh": "⚠️ 还没有配置交易所。请在 config.yaml 的 exchanges 中添加交易所 API Key。",
+		"en": "⚠️ No exchange configured. Add exchange API keys in config.yaml.",
+	},
+	"trade_usage": {
+		"zh": "❓ 用法: `/buy BTC 0.01` 或 `/sell ETH 0.5 3x`",
+		"en": "❓ Usage: `/buy BTC 0.01` or `/sell ETH 0.5 3x`",
+	},
+	"invalid_quantity": {
+		"zh": "❓ 无效数量: %s",
+		"en": "❓ Invalid quantity: %s",
+	},
+	"specify_quantity": {
+		"zh": "❓ 请指定数量: `/buy BTC 0.01`",
+		"en": "❓ Please specify quantity: `/buy BTC 0.01`",
+	},
+	"confirm_trade": {
+		"zh": "⚡ *确认交易*\n\n• 操作: %s\n• 交易对: %s\n• 数量: %.6f\n• 杠杆: %dx\n• 交易所: %s\n\n回复 '确认' 执行交易。",
+		"en": "⚡ *Confirm Trade*\n\n• Action: %s\n• Symbol: %s\n• Quantity: %.6f\n• Leverage: %dx\n• Exchange: %s\n\nReply 'yes' to execute.",
+	},
+	"trade_executed": {
+		"zh": "✅ *交易已执行！*\n\n• %s %s\n• 数量: %.6f\n• 杠杆: %dx\n• 交易所: %s\n• 结果: %v",
+		"en": "✅ *Trade Executed!*\n\n• %s %s\n• Qty: %.6f\n• Leverage: %dx\n• Exchange: %s\n• Result: %v",
+	},
+	"no_pending": {
+		"zh": "没有待确认的交易",
+		"en": "no pending trade",
+	},
+	"analysis_signal": {
+		"zh": "🔍 *%s/USDT 分析*\n\n信号: %s\n置信度: %.0f%%\n\n%s",
+		"en": "🔍 *%s/USDT Analysis*\n\nSignal: %s\nConfidence: %.0f%%\n\n%s",
+	},
+	"stop_loss": {
+		"zh": "\n\n🛑 止损: $%.2f",
+		"en": "\n\n🛑 Stop Loss: $%.2f",
+	},
+	"take_profit": {
+		"zh": "\n🎯 止盈: $%.2f",
+		"en": "\n🎯 Take Profit: $%.2f",
+	},
+	"settings": {
+		"zh": "⚙️ *设置*\n\n• 语言: %s\n• 模型: %s\n• 提供商: %s\n• 交易所: %d 个已配置",
+		"en": "⚙️ *Settings*\n\n• Language: %s\n• Model: %s\n• Provider: %s\n• Exchanges: %d configured",
+	},
+	"status_title": {
+		"zh": "📊 *NOFXi 状态*\n\n• Agent: %s\n• 模型: %s\n• 提供商: %s\n• 记忆: ✅ 在线\n• 执行: %s\n• 监控: %d 个交易对\n• 时间: %s",
+		"en": "📊 *NOFXi Status*\n\n• Agent: %s\n• Model: %s\n• Provider: %s\n• Memory: ✅ Online\n• Execution: %s\n• Watching: %d symbols\n• Time: %s",
+	},
+	"bridge_connected": {
+		"zh": "✅ 已连接",
+		"en": "✅ Connected",
+	},
+	"bridge_disconnected": {
+		"zh": "❌ 未连接",
+		"en": "❌ Not connected",
+	},
+	"ai_timeout": {
+		"zh": "⏱️ AI 响应超时，请稍后再试。",
+		"en": "⏱️ AI response timed out, please try again.",
+	},
+	"system_prompt": {
+		"zh": `你是 NOFXi，一个基于 NOFX 构建的 AI 交易 Agent。
+
+你的能力：
+- 市场分析和交易建议
+- 实时持仓和余额监控
+- 交易执行（开仓/平仓）
+- 价格提醒和行情监控
+- 风险管理建议
+
+支持多家交易所：Binance、OKX、Bybit、Bitget、KuCoin、Gate、Hyperliquid 等。
+
+简洁、自信、专注交易。使用交易相关的 emoji。
+用中文回复。
+当前时间: %s`,
+		"en": `You are NOFXi, an AI trading agent built on NOFX.
+
+Your capabilities:
+- Market analysis and trading recommendations
+- Real-time position and balance monitoring
+- Trade execution (open/close positions)
+- Price alerts and market monitoring
+- Risk management advice
+
+Supports multiple exchanges: Binance, OKX, Bybit, Bitget, KuCoin, Gate, Hyperliquid, etc.
+
+Be concise, confident, and action-oriented. Use trading emojis.
+Respond in English.
+Current time: %s`,
+	},
+}
+
+// msg returns the localized message for the given key and language.
+func msg(lang, key string) string {
+	if m, ok := messages[key]; ok {
+		if s, ok := m[lang]; ok {
+			return s
+		}
+		if s, ok := m["en"]; ok {
+			return s
+		}
+	}
+	return key
+}