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/components/BacktestPanel.vue

@@ -9,6 +9,7 @@ import StrategyForm from './StrategyForm.vue'
 defineProps({
   strategies: { type: Array, required: true },
   status: { type: Object, required: true },
+  loading: { type: Boolean, default: false },
 })
 
 const running = ref(false)
@@ -44,6 +45,7 @@ async function run(payload) {
       :busy="running"
       :symbol="status.symbol"
       :timeframe="status.timeframe"
+      :loading="loading"
       submit-label="Run backtest"
       @submit="run"
     />