Fix page refresh killing running simulations

SimulationView was stopping running simulations on every mount,
including page refreshes. Now only stops when user explicitly
navigates back from Step 3 via fromStep3 query param.

frontend/src/views/SimulationRunView.vue

@@ -188,8 +188,8 @@ const handleGoBack = async () => {
     addLog(`Failed to check simulation status: ${err.message}`)
   }
   
-  // 返回到 Step 2 (环境搭建)
+  // 返回到 Step 2 (环境搭建), with flag so Step 2 knows to stop the sim
-  router.push({ name: 'Simulation', params: { simulationId: currentSimulationId.value } })
+  router.push({ name: 'Simulation', params: { simulationId: currentSimulationId.value }, query: { fromStep3: '1' } })
 }
 
 const handleNextStep = () => {

frontend/src/views/SimulationView.vue

@@ -289,8 +289,13 @@ const refreshGraph = () => {
 onMounted(async () => {
   addLog('SimulationView initialized')
 
-  // 检查并关闭正在运行的模拟（用户从 Step 3 返回时）
+  // Only stop running simulation if user explicitly navigated back from Step 3
+  // (indicated by 'fromStep3' query param), not on page refresh
+  if (route.query.fromStep3) {
     await checkAndStopRunningSimulation()
+    // Clean up the query param
+    router.replace({ ...route, query: {} })
+  }
 
   // 加载模拟数据
   loadSimulationData()