feat(cryptotail): 尾盘监控双连接与分时图优化

- 监控 WebSocket 拆分为当前周期连接与下一周期连接，周期切换时关闭过期连接并新建下一周期
- 下一周期市场未创建时也建立第二条空连接，保证始终两条连接
- refreshSubscription 增加 Mutex 防重入，避免周期结束时定时器与消息同时触发导致重复执行
- 修复 initMonitor/buildPushData 中 getCurrentOpenClose、spreadMode/spreadValue 等 API 与实体字段引用
- 移除重复的 buildSubscriptionMap、buildPushData 等方法，修复 StrategyPriceData.periodStartUnix
- 前端分时图：市场价折线增加 connectNulls，新周期默认 0.5 价格展示
- 多语言与监控页入口、API 类型与 WebSocket 订阅集成

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/services/api.ts

@@ -497,7 +497,9 @@ export const apiService = {
     marketOptions: () =>
       apiClient.post<ApiResponse<import('../types').CryptoTailMarketOptionDto[]>>('/crypto-tail-strategy/market-options', {}),
     autoMinSpread: (data: { intervalSeconds: number }) =>
-      apiClient.post<ApiResponse<import('../types').CryptoTailAutoMinSpreadResponse>>('/crypto-tail-strategy/auto-min-spread', data)
+      apiClient.post<ApiResponse<import('../types').CryptoTailAutoMinSpreadResponse>>('/crypto-tail-strategy/auto-min-spread', data),
+    monitorInit: (strategyId: number) =>
+      apiClient.post<ApiResponse<import('../types').CryptoTailMonitorInitResponse>>('/crypto-tail-strategy/monitor/init', { strategyId })
   },
   
   /**