fix(frontend): 修复 now 变量重复声明导致的空白页面

- 从 runtimeStore 移除 now（应从 uiStore 获取）
- 修复重复声明错误

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/App.jsx

@@ -76,10 +76,10 @@ function ViewLoadingFallback({ label = '加载中...' }) {
 
 export default function LiveTradingApp() {
   // Connection & system state - from runtimeStore
-  const { isConnected, setIsConnected, connectionStatus, setConnectionStatus, systemStatus, setSystemStatus, currentDate, setCurrentDate, progress, setProgress, now, setNow } = useRuntimeStore();
+  const { isConnected, setIsConnected, connectionStatus, setConnectionStatus, systemStatus, setSystemStatus, currentDate, setCurrentDate, progress, setProgress } = useRuntimeStore();
 
   // View toggle: 'traders' | 'room' | 'explain' | 'chart' | 'statistics' | 'runtime'
-  const { currentView, setCurrentView, chartTab, setChartTab, isInitialAnimating, setIsInitialAnimating, lastUpdate, setLastUpdate, isUpdating, setIsUpdating } = useUIStore();
+  const { currentView, setCurrentView, chartTab, setChartTab, isInitialAnimating, setIsInitialAnimating, lastUpdate, setLastUpdate, isUpdating, setIsUpdating, now, setNow } = useUIStore();
 
   // Chart data - from portfolioStore
   const { portfolioData, setPortfolioData, holdings, setHoldings, trades, setTrades, stats, setStats, leaderboard, setLeaderboard } = usePortfolioStore();