fix: 重启后加仓数据恢复 + scale_count/amount_usd 实时持久化

- restoreOpenPositions 从 orders 表 count 加仓次数，正确设置 ScaleLevels 和 AmountUSD
- 加仓时立即持久化 amount_usd 和 scale_count 到 trades 表（不等到平仓）
- PnL 图表支持单笔交易显示（原点+累计PnL连线）

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/App.jsx

@@ -651,6 +651,11 @@ function PnlChart() {
     // Compute cumulative PnL
     const points = []
     let cum = 0
+    // Prepend a zero point so single-trade chart still draws
+    if (data.length > 0) {
+      const t0 = new Date(data[0].ClosedAt).getTime() - 1000
+      points.push({ x: t0, y: 0 })
+    }
     for (const t of data) {
       cum += 2 * (t.AmountUSD || 0) * (t.NetPnl || 0) / 100
       points.push({ x: new Date(t.ClosedAt).getTime(), y: cum })
@@ -756,7 +761,7 @@ function PnlChart() {
     <section className="card card-wide" id="pnl-chart-card">
       <h2>📈 总PnL成长曲线 <span className="text-dim" style={{fontSize:11}}>{data.length > 0 ? `$${totalPnl.toFixed(2)}` : ''}</span></h2>
       <div className="chart-container" style={{height:260}}>
-        {data.length < 2 ? (
+        {data.length < 1 ? (
           <div className="loading" style={{paddingTop:100}}>暂无数据...</div>
         ) : (
           <canvas ref={canvasRef} />