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/App.tsx

@@ -35,6 +35,7 @@ import Announcements from './pages/Announcements'
 import BacktestList from './pages/BacktestList'
 import BacktestDetail from './pages/BacktestDetail'
 import CryptoTailStrategyList from './pages/CryptoTailStrategyList'
+import CryptoTailMonitor from './pages/CryptoTailMonitor'
 import { wsManager } from './services/websocket'
 import type { OrderPushMessage } from './types'
 import { apiService } from './services/api'
@@ -252,6 +253,7 @@ function App() {
           <Route path="/templates/edit/:id" element={<ProtectedRoute><TemplateEdit /></ProtectedRoute>} />
           <Route path="/copy-trading" element={<ProtectedRoute><CopyTradingList /></ProtectedRoute>} />
           <Route path="/crypto-tail-strategy" element={<ProtectedRoute><CryptoTailStrategyList /></ProtectedRoute>} />
+          <Route path="/crypto-tail-monitor" element={<ProtectedRoute><CryptoTailMonitor /></ProtectedRoute>} />
           <Route path="/copy-trading/statistics/:copyTradingId" element={<ProtectedRoute><CopyTradingStatistics /></ProtectedRoute>} />
           {/* 保留旧路由以保持向后兼容 */}
           <Route path="/copy-trading/orders/buy/:copyTradingId" element={<ProtectedRoute><CopyTradingBuyOrders /></ProtectedRoute>} />