Retry the strategy list and make collapse obvious

- The strategy/preset lists were loaded once with failures swallowed, so a
  single failed first request left the strategy dropdown empty forever. They
  are now retried every 3s until populated.
- Clearer collapse affordance: hover highlight, a hide/show hint, and a
  tooltip; Backtest and Parameter sweep start collapsed so the strategy
  selector and status are immediately visible.
- Adds an integration test that the strategy dropdown is populated and the
  panel actually collapses.

frontend/src/components/BacktestPanel.vue

@@ -37,7 +37,7 @@ async function run(payload) {
 </script>
 
 <template>
-  <CollapsiblePanel title="Backtest" storage-key="backtest" :summary="summary">
+  <CollapsiblePanel title="Backtest" storage-key="backtest" :summary="summary" :default-open="false">
     <div class="space-y-3">
       <StrategyForm
       :strategies="strategies"