Show a loading state in the strategy dropdown

While the strategy list is being fetched the dropdown now shows a disabled
"Loading strategies…" option instead of an empty select, and "No strategies
available" once loading finishes with no results. Applies to the strategy,
backtest and sweep panels via a loading prop. Adds tests.

frontend/src/App.vue

@@ -29,6 +29,7 @@ const status = ref({
 })
 
 const strategies = ref([])
+const strategiesLoading = ref(true)
 const presets = ref([])
 const positions = ref([])
 const orders = ref([])
@@ -71,7 +72,12 @@ async function refreshAll() {
 }
 
 async function loadStrategies() {
-  strategies.value = await api.getStrategies()
+  strategiesLoading.value = true
+  try {
+    strategies.value = await api.getStrategies()
+  } finally {
+    strategiesLoading.value = false
+  }
 }
 
 async function loadPresets() {
@@ -265,6 +271,7 @@ onUnmounted(() => {
       :status="status"
       :busy="busy"
       :presets="presets"
+      :loading="strategiesLoading"
       @start="handleStart"
       @stop="handleStop"
       @kill="handleKill"
@@ -272,8 +279,8 @@ onUnmounted(() => {
       @delete-preset="handleDeletePreset"
     />
 
-    <BacktestPanel :strategies="strategies" :status="status" />
-    <SweepPanel :strategies="strategies" :status="status" />
+    <BacktestPanel :strategies="strategies" :status="status" :loading="strategiesLoading" />
+    <SweepPanel :strategies="strategies" :status="status" :loading="strategiesLoading" />
 
     <div class="grid gap-4 lg:grid-cols-2">
       <EquityChart :points="equityPoints" />