fix(portfolio-backtest): 净值曲线 echarts 空白 - watch漏触发手动兜底

根因(浏览器实证): onSubmit 异步轮询 done 后 result/equityCurve 同帧赋值,
v-if result 块(含 chart-box div)此帧才挂载, watch(equityCurve, flush:post)
在该时序下漏触发 renderEquity → echarts 从未 init(chart-box 无 _echarts_instance_)。
手动 echarts.init+setOption 实证容器/echarts/数据全正常(37点),纯 watch 触发遗漏。

修复: equityCurve 赋值后 await nextTick()(等 chart-box 挂载)+ 手动 renderEquity()兜底。
watch 保留(双保险, ensureChart 幂等)。

frontend/src/views/backtest/PortfolioBacktest.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { ref, reactive, onMounted, onUnmounted, watch } from 'vue'
+import { ref, reactive, onMounted, onUnmounted, watch, nextTick } from 'vue'
 import { ElMessage } from 'element-plus'
 import type { EChartsCoreOption } from 'echarts'
 import { useChart } from '@/composables/useChart'
@@ -126,6 +126,11 @@ async function onSubmit(): Promise<void> {
     trades.value = r.trades || []
     metrics.value = r.metrics || null
     period.value = r.period || null
+    // v-if result 块(含 chart-box div)在此帧才挂载, watch(equityCurve, flush:post)
+    // 在异步轮询 done 后赋值时序下会漏触发 echarts.init → 净值曲线空白。
+    // nextTick 等 chart-box 挂载后手动渲染兜底。
+    await nextTick()
+    renderEquity()
     ElMessage.success(
       `回测完成: ${r.period?.trading_days ?? 0} 交易日, 选股 ${(r.stocks_selected || []).length} 只`,
     )